How to fix Error 6520 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 6520 – .NET Framework Error During Statement Execution

Why This Error Happens

Error 6520 occurs when a SQLCLR (SQL Common Language Runtime) integration — such as a CLR stored procedure, function, trigger, or user-defined type — throws an unhandled .NET exception during execution. This is typically caused by corrupted or outdated CLR assemblies, a mismatch between the SQL Server CLR runtime version and the .NET Framework version installed on Windows Server 2025 / Windows 11, or insufficient PERMISSION_SET rights (SAFE, EXTERNAL_ACCESS, UNSAFE) assigned to the assembly. It can also surface due to resource exhaustion (memory pressure) within the CLR host, causing managed code to fail mid-execution.

Step-by-Step Resolution Guide

Step 1: Identify the Failing Assembly and Capture the Inner Exception

  • Query sys.assemblies and sys.assembly_modules to locate the CLR object referenced in the error message.
  • Enable detailed error capture by running the failing statement with SET XACT_ABORT ON; to surface the full .NET stack trace.
  • Check the SQL Server Error Log (C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG) for the underlying System.Exception type (e.g., NullReferenceException, OutOfMemoryException).

Step 2: Verify and Repair the .NET Framework / CLR Configuration

  • Confirm CLR integration is enabled by running:
  EXEC sp_configure 'clr enabled', 1;
  RECONFIGURE;
  • On Windows Server 2025 / Windows 11, open Control Panel > Programs > Turn Windows features on or off and ensure .NET Framework 4.8.x (or the version SQL Server 2025 requires) is fully enabled, not just partially installed.
  • Run sfc /scannow from an elevated Command Prompt to repair corrupted system/.NET binaries that may be causing the managed exception.

Step 3: Re-register and Rebuild the Problematic Assembly

  • Drop and re-add the assembly with correct permissions:
  ALTER ASSEMBLY [YourAssemblyName]
  FROM 'C:\SQLAssemblies\YourAssembly.dll'
  WITH PERMISSION_SET = EXTERNAL_ACCESS;
  • If the assembly uses UNSAFE, confirm the database has TRUSTWORTHY ON or that the assembly is properly signed with a certificate/asymmetric key, since Windows Server 2025 enforces stricter CLR strict security by default.
  • Rebuild the DLL against the exact .NET Framework version matching the SQL Server 2025 CLR host.

Step 4: Allocate Sufficient Memory for the CLR Host

  • Open SQL Server Management Studio (SSMS), right-click the instance, and go to Properties > Memory.
  • Increase Maximum server memory to leave adequate headroom for the CLR virtual machine, especially if running on a shared Windows Server 2025 host with limited RAM.
  • Restart the SQL Server service via services.msc or NET STOP MSSQLSERVER && NET START MSSQLSERVER to apply changes.

Alternative or Verification Steps

  • Re-execute the original failing statement/stored procedure in SSMS and confirm no Error 6520 is raised; check sys.dm_exec_requests to ensure the session completes without rollback.
  • If the issue persists, as a fallback, rewrite the CLR logic as a native T-SQL procedure/function to eliminate the .NET dependency entirely.