diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2d6e4d3..afd47ee 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [2.40.1] - 2026-03-21
+
+### Fixed
+
+- **`n8n_manage_datatable` row operations broken by MCP transport serialization**: `data` parameter received as string instead of JSON — added `z.preprocess` coercers for array/object/filter params
+- **`n8n_manage_datatable` filter/sortBy URL encoding**: n8n API requires URL-encoded query params — added `encodeURIComponent()` for filter and sortBy in getRows and deleteRows
+- **`json` column type rejected by n8n API**: Removed `json` from column type enum (n8n only accepts string/number/boolean/date)
+- **Garbled 404 error messages**: Fixed `N8nNotFoundError` constructor — API error messages are now passed through cleanly instead of being wrapped in "Resource with ID ... not found"
+
+Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en
+
 ## [2.40.0] - 2026-03-21
 
 ### Changed
diff --git a/package.json b/package.json
index f9271ea..9cde240 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp",
-  "version": "2.40.0",
+  "version": "2.40.1",
   "description": "Integration between n8n workflow automation and Model Context Protocol (MCP)",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",
diff --git a/src/mcp/handlers-n8n-manager.ts b/src/mcp/handlers-n8n-manager.ts
index a2dc38a..9b2e508 100644
--- a/src/mcp/handlers-n8n-manager.ts
+++ b/src/mcp/handlers-n8n-manager.ts
@@ -2716,7 +2716,7 @@ const createTableSchema = z.object({
   name: z.string().min(1, 'Table name cannot be empty'),
   columns: z.array(z.object({
     name: z.string().min(1, 'Column name cannot be empty'),
-    type: z.enum(['string', 'number', 'boolean', 'date', 'json']).optional(),
+    type: z.enum(['string', 'number', 'boolean', 'date']).optional(),
   })).optional(),
 });
 
@@ -2729,29 +2729,40 @@ const updateTableSchema = tableIdSchema.extend({
   name: z.string().min(1, 'New table name cannot be empty'),
 });
 
+// MCP transports may serialize JSON objects/arrays as strings.
+// Parse them back, but return the original value on failure so Zod reports a proper type error.
+function tryParseJson(val: unknown): unknown {
+  if (typeof val !== 'string') return val;
+  try { return JSON.parse(val); } catch { return val; }
+}
+
+const coerceJsonArray = z.preprocess(tryParseJson, z.array(z.record(z.unknown())));
+const coerceJsonObject = z.preprocess(tryParseJson, z.record(z.unknown()));
+const coerceJsonFilter = z.preprocess(tryParseJson, dataTableFilterSchema);
+
 const getRowsSchema = tableIdSchema.extend({
   limit: z.number().min(1).max(100).optional(),
   cursor: z.string().optional(),
-  filter: z.union([dataTableFilterSchema, z.string()]).optional(),
+  filter: z.union([coerceJsonFilter, z.string()]).optional(),
   sortBy: z.string().optional(),
   search: z.string().optional(),
 });
 
 const insertRowsSchema = tableIdSchema.extend({
-  data: z.array(z.record(z.unknown())).min(1, 'At least one row is required'),
+  data: coerceJsonArray.pipe(z.array(z.record(z.unknown())).min(1, 'At least one row is required')),
   returnType: z.enum(['count', 'id', 'all']).optional(),
 });
 
 // Shared schema for update/upsert (identical structure)
 const mutateRowsSchema = tableIdSchema.extend({
-  filter: dataTableFilterSchema,
-  data: z.record(z.unknown()),
+  filter: coerceJsonFilter,
+  data: coerceJsonObject,
   returnData: z.boolean().optional(),
   dryRun: z.boolean().optional(),
 });
 
 const deleteRowsSchema = tableIdSchema.extend({
-  filter: dataTableFilterSchema,
+  filter: coerceJsonFilter,
   returnData: z.boolean().optional(),
   dryRun: z.boolean().optional(),
 });
@@ -2847,10 +2858,14 @@ export async function handleDeleteTable(args: unknown, context?: InstanceContext
 export async function handleGetRows(args: unknown, context?: InstanceContext): Promise<McpToolResponse> {
   try {
     const client = ensureApiConfigured(context);
-    const { tableId, filter, ...params } = getRowsSchema.parse(args);
+    const { tableId, filter, sortBy, ...params } = getRowsSchema.parse(args);
     const queryParams: Record<string, unknown> = { ...params };
     if (filter) {
-      queryParams.filter = typeof filter === 'string' ? filter : JSON.stringify(filter);
+      const filterStr = typeof filter === 'string' ? filter : JSON.stringify(filter);
+      queryParams.filter = encodeURIComponent(filterStr);
+    }
+    if (sortBy) {
+      queryParams.sortBy = encodeURIComponent(sortBy);
     }
     const result = await client.getDataTableRows(tableId, queryParams as any);
     return {
@@ -2916,7 +2931,7 @@ export async function handleDeleteRows(args: unknown, context?: InstanceContext)
     const client = ensureApiConfigured(context);
     const { tableId, filter, ...params } = deleteRowsSchema.parse(args);
     const queryParams = {
-      filter: JSON.stringify(filter),
+      filter: encodeURIComponent(JSON.stringify(filter)),
       ...params,
     };
     const result = await client.deleteDataTableRows(tableId, queryParams as any);
diff --git a/src/mcp/tool-docs/workflow_management/n8n-manage-datatable.ts b/src/mcp/tool-docs/workflow_management/n8n-manage-datatable.ts
index 73214d9..e530cc8 100644
--- a/src/mcp/tool-docs/workflow_management/n8n-manage-datatable.ts
+++ b/src/mcp/tool-docs/workflow_management/n8n-manage-datatable.ts
@@ -42,7 +42,7 @@ export const n8nManageDatatableDoc: ToolDocumentation = {
       action: { type: 'string', required: true, description: 'Operation to perform' },
       tableId: { type: 'string', required: false, description: 'Data table ID (required for all except createTable and listTables)' },
       name: { type: 'string', required: false, description: 'For createTable/updateTable: table name' },
-      columns: { type: 'array', required: false, description: 'For createTable: column definitions [{name, type?}]. Types: string, number, boolean, date, json' },
+      columns: { type: 'array', required: false, description: 'For createTable: column definitions [{name, type?}]. Types: string, number, boolean, date' },
       data: { type: 'array|object', required: false, description: 'For insertRows: array of row objects. For updateRows/upsertRows: object with column values' },
       filter: { type: 'object', required: false, description: 'Filter: {type?: "and"|"or", filters: [{columnName, condition, value}]}' },
       limit: { type: 'number', required: false, description: 'For listTables/getRows: max results (1-100)' },
diff --git a/src/mcp/tools-n8n-manager.ts b/src/mcp/tools-n8n-manager.ts
index cf82442..16bea4d 100644
--- a/src/mcp/tools-n8n-manager.ts
+++ b/src/mcp/tools-n8n-manager.ts
@@ -627,7 +627,7 @@ export const n8nManagementTools: ToolDefinition[] = [
             type: 'object',
             properties: {
               name: { type: 'string' },
-              type: { type: 'string', enum: ['string', 'number', 'boolean', 'date', 'json'] },
+              type: { type: 'string', enum: ['string', 'number', 'boolean', 'date'] },
             },
             required: ['name'],
           },
diff --git a/src/types/n8n-api.ts b/src/types/n8n-api.ts
index 6d3d490..c8ce9bb 100644
--- a/src/types/n8n-api.ts
+++ b/src/types/n8n-api.ts
@@ -459,13 +459,13 @@ export interface ErrorSuggestion {
 // Data Table types
 export interface DataTableColumn {
   name: string;
-  type?: 'string' | 'number' | 'boolean' | 'date' | 'json';
+  type?: 'string' | 'number' | 'boolean' | 'date';
 }
 
 export interface DataTableColumnResponse {
   id: string;
   name: string;
-  type: 'string' | 'number' | 'boolean' | 'date' | 'json';
+  type: 'string' | 'number' | 'boolean' | 'date';
   index: number;
 }
 
diff --git a/src/utils/n8n-errors.ts b/src/utils/n8n-errors.ts
index d8a7c5b..710b1ac 100644
--- a/src/utils/n8n-errors.ts
+++ b/src/utils/n8n-errors.ts
@@ -22,8 +22,10 @@ export class N8nAuthenticationError extends N8nApiError {
 }
 
 export class N8nNotFoundError extends N8nApiError {
-  constructor(resource: string, id?: string) {
-    const message = id ? `${resource} with ID ${id} not found` : `${resource} not found`;
+  constructor(messageOrResource: string, id?: string) {
+    // If id is provided, format as "resource with ID id not found"
+    // Otherwise, use messageOrResource as-is (it's already a complete message from the API)
+    const message = id ? `${messageOrResource} with ID ${id} not found` : messageOrResource;
     super(message, 404, 'NOT_FOUND');
     this.name = 'N8nNotFoundError';
   }
@@ -70,7 +72,7 @@ export function handleN8nApiError(error: unknown): N8nApiError {
         case 401:
           return new N8nAuthenticationError(message);
         case 404:
-          return new N8nNotFoundError('Resource', message);
+          return new N8nNotFoundError(message || 'Resource');
         case 400:
           return new N8nValidationError(message, data);
         case 429:
diff --git a/tests/unit/mcp/handlers-manage-datatable.test.ts b/tests/unit/mcp/handlers-manage-datatable.test.ts
index fe28414..19885ac 100644
--- a/tests/unit/mcp/handlers-manage-datatable.test.ts
+++ b/tests/unit/mcp/handlers-manage-datatable.test.ts
@@ -429,12 +429,12 @@ describe('Data Table Handlers (n8n_manage_datatable)', () => {
 
       expect(mockApiClient.getDataTableRows).toHaveBeenCalledWith('dt-1', {
         limit: 50,
-        sortBy: 'name:asc',
+        sortBy: encodeURIComponent('name:asc'),
         search: 'john',
       });
     });
 
-    it('should serialize object filter to JSON string', async () => {
+    it('should serialize object filter to URL-encoded JSON string', async () => {
       mockApiClient.getDataTableRows.mockResolvedValue({ data: [], nextCursor: null });
 
       const objectFilter = {
@@ -448,20 +448,21 @@ describe('Data Table Handlers (n8n_manage_datatable)', () => {
       });
 
       expect(mockApiClient.getDataTableRows).toHaveBeenCalledWith('dt-1', {
-        filter: JSON.stringify(objectFilter),
+        filter: encodeURIComponent(JSON.stringify(objectFilter)),
       });
     });
 
-    it('should pass through string filter as-is', async () => {
+    it('should URL-encode string filter', async () => {
       mockApiClient.getDataTableRows.mockResolvedValue({ data: [], nextCursor: null });
 
+      const filterStr = '{"type":"and","filters":[]}';
       await handlers.handleGetRows({
         tableId: 'dt-1',
-        filter: '{"type":"and","filters":[]}',
+        filter: filterStr,
       });
 
       expect(mockApiClient.getDataTableRows).toHaveBeenCalledWith('dt-1', {
-        filter: '{"type":"and","filters":[]}',
+        filter: encodeURIComponent(filterStr),
       });
     });
   });
@@ -680,11 +681,11 @@ describe('Data Table Handlers (n8n_manage_datatable)', () => {
         message: 'Rows deleted successfully',
       });
       expect(mockApiClient.deleteDataTableRows).toHaveBeenCalledWith('dt-1', {
-        filter: JSON.stringify({ type: 'and', ...filter }),
+        filter: encodeURIComponent(JSON.stringify({ type: 'and', ...filter })),
       });
     });
 
-    it('should serialize filter object to JSON string for API call', async () => {
+    it('should URL-encode serialized filter for API call', async () => {
       mockApiClient.deleteDataTableRows.mockResolvedValue({ deletedCount: 1 });
 
       const filter = {
@@ -698,7 +699,7 @@ describe('Data Table Handlers (n8n_manage_datatable)', () => {
       await handlers.handleDeleteRows({ tableId: 'dt-1', filter });
 
       expect(mockApiClient.deleteDataTableRows).toHaveBeenCalledWith('dt-1', {
-        filter: JSON.stringify(filter),
+        filter: encodeURIComponent(JSON.stringify(filter)),
       });
     });
 
@@ -719,7 +720,7 @@ describe('Data Table Handlers (n8n_manage_datatable)', () => {
       expect(result.success).toBe(true);
       expect(result.message).toBe('Dry run: rows matched for deletion (no changes applied)');
       expect(mockApiClient.deleteDataTableRows).toHaveBeenCalledWith('dt-1', {
-        filter: JSON.stringify({ type: 'and', ...filter }),
+        filter: encodeURIComponent(JSON.stringify({ type: 'and', ...filter })),
         dryRun: true,
       });
     });
diff --git a/tests/unit/mcp/handlers-n8n-manager.test.ts b/tests/unit/mcp/handlers-n8n-manager.test.ts
index 1207078..54dbe0f 100644
--- a/tests/unit/mcp/handlers-n8n-manager.test.ts
+++ b/tests/unit/mcp/handlers-n8n-manager.test.ts
@@ -100,6 +100,16 @@ describe('handlers-n8n-manager', () => {
       listExecutions: vi.fn(),
       deleteExecution: vi.fn(),
       healthCheck: vi.fn(),
+      createDataTable: vi.fn(),
+      listDataTables: vi.fn(),
+      getDataTable: vi.fn(),
+      updateDataTable: vi.fn(),
+      deleteDataTable: vi.fn(),
+      getDataTableRows: vi.fn(),
+      insertDataTableRows: vi.fn(),
+      updateDataTableRows: vi.fn(),
+      upsertDataTableRow: vi.fn(),
+      deleteDataTableRows: vi.fn(),
     };
 
     // Setup mock repository
diff --git a/tests/unit/services/n8n-api-client.test.ts b/tests/unit/services/n8n-api-client.test.ts
index 0a87089..3ca8eaa 100644
--- a/tests/unit/services/n8n-api-client.test.ts
+++ b/tests/unit/services/n8n-api-client.test.ts
@@ -297,7 +297,7 @@ describe('N8nApiClient', () => {
         expect.fail('Should have thrown an error');
       } catch (err) {
         expect(err).toBeInstanceOf(N8nNotFoundError);
-        expect((err as N8nNotFoundError).message).toContain('not found');
+        expect((err as N8nNotFoundError).message.toLowerCase()).toContain('not found');
         expect((err as N8nNotFoundError).statusCode).toBe(404);
       }
     });
@@ -380,7 +380,7 @@ describe('N8nApiClient', () => {
         expect.fail('Should have thrown an error');
       } catch (err) {
         expect(err).toBeInstanceOf(N8nNotFoundError);
-        expect((err as N8nNotFoundError).message).toContain('not found');
+        expect((err as N8nNotFoundError).message.toLowerCase()).toContain('not found');
         expect((err as N8nNotFoundError).statusCode).toBe(404);
       }
     });
@@ -432,7 +432,7 @@ describe('N8nApiClient', () => {
         expect.fail('Should have thrown an error');
       } catch (err) {
         expect(err).toBeInstanceOf(N8nNotFoundError);
-        expect((err as N8nNotFoundError).message).toContain('not found');
+        expect((err as N8nNotFoundError).message.toLowerCase()).toContain('not found');
         expect((err as N8nNotFoundError).statusCode).toBe(404);
       }
     });
@@ -501,7 +501,7 @@ describe('N8nApiClient', () => {
         expect.fail('Should have thrown an error');
       } catch (err) {
         expect(err).toBeInstanceOf(N8nNotFoundError);
-        expect((err as N8nNotFoundError).message).toContain('not found');
+        expect((err as N8nNotFoundError).message.toLowerCase()).toContain('not found');
         expect((err as N8nNotFoundError).statusCode).toBe(404);
       }
     });
@@ -1278,7 +1278,7 @@ describe('N8nApiClient', () => {
         expect.fail('Should have thrown an error');
       } catch (err) {
         expect(err).toBeInstanceOf(N8nNotFoundError);
-        expect((err as N8nNotFoundError).message).toContain('not found');
+        expect((err as N8nNotFoundError).message.toLowerCase()).toContain('not found');
         expect((err as N8nNotFoundError).statusCode).toBe(404);
       }
     });