How to fix Error 6533 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 6533 (AppDomain Unloaded Due to Memory Pressure)

Why This Error Happens

This error occurs when SQL Server’s CLR (Common Language Runtime) integration runs out of memory while executing a .NET assembly, triggering the escalation policy to forcibly unload the AppDomain to protect server stability. This is typically caused by excessive memory consumption from CLR-based stored procedures, functions, or triggers, often combined with insufficient memory allocation to SQL Server or competing memory pressure from other processes on the Windows Server 2025 host. In some cases, memory fragmentation within the CLR heap or a memory leak in custom assembly code can also trigger this escalation even when overall system memory appears sufficient.

Step-by-Step Resolution Guide

Step 1: Verify Current Memory Configuration

  • Open SQL Server Management Studio (SSMS) and run EXEC sp_configure 'max server memory' to check current allocation.
  • Ensure “max server memory” leaves adequate headroom for the OS (typically reserve 4-8 GB for Windows Server 2025 itself).
  • Check Windows Task Manager or Performance Monitor (perfmon.exe) for overall system memory pressure during the error timeframe.

Step 2: Review and Isolate the Offending CLR Assembly

  • Query sys.dm_clr_loaded_assemblies and sys.dm_clr_appdomains to identify which AppDomain and assembly triggered the unload.
  • Check SQL Server error logs located at C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG for the specific assembly name referenced in the %.*ls placeholders.
  • Review the custom .NET code for unmanaged memory leaks, large object allocations, or missing Dispose() calls on IDisposable objects.

Step 3: Adjust SQL Server Memory and CLR Settings

  • Increase “max server memory” via: EXEC sp_configure 'show advanced options', 1; RECONFIGURE; EXEC sp_configure 'max server memory', <value_in_MB>; RECONFIGURE;
  • Enable “clr strict security” validation by confirming assemblies are signed and trusted: EXEC sp_configure 'clr strict security', 1; RECONFIGURE;
  • Restart the SQL Server service via services.msc or NET STOP MSSQLSERVER && NET START MSSQLSERVER to apply changes.

Step 4: Patch or Update the CLR Assembly

  • Recompile the assembly targeting the latest .NET runtime compatible with SQL Server 2025.
  • Redeploy using CREATE ASSEMBLY with the WITH PERMISSION_SET = SAFE clause unless EXTERNAL_ACCESS is explicitly required.
  • Test the assembly under load using SQL Server Profiler or Extended Events to monitor memory consumption patterns.

Step 5: Apply Latest SQL Server 2025 Cumulative Update

  • Navigate to Microsoft Update Catalog and download the latest Cumulative Update (CU) for SQL Server 2025.
  • Run the installer with administrative privileges and follow the setup wizard to patch known CLR memory management bugs.
  • Reboot the Windows Server 2025 instance if prompted post-installation.

Alternative or Verification Steps

  • To verify the fix, re-run the workload that previously triggered the error and monitor sys.dm_os_memory_clerks for the MEMORYCLERK_SQLCLR clerk to confirm memory usage remains stable.
  • If the error persists, consider disabling CLR integration entirely as a temporary workaround via EXEC sp_configure 'clr enabled', 0; RECONFIGURE; while a permanent code fix is developed.