How to fix Error 5546 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: FILESTREAM Error 5546 – “Failed to Connect to Kernel Driver”

Why This Error Happens

This error occurs when the SQL Server FILESTREAM feature is enabled at the service level but the underlying kernel-mode driver (fltmgr.sys filter or the dedicated FILESTREAM driver) is not loaded, disabled, or improperly registered in Windows Server 2025 / Windows 11. It is typically caused by the FILESTREAM Windows service being disabled, a mismatch between the SQL Server configuration and the OS-level feature enablement, or corruption/removal of the driver files during an OS or SQL Server upgrade. In some cases, Group Policy or security software blocks the minifilter driver from attaching to the volume.

Step-by-Step Resolution Guide

Step 1: Verify the FILESTREAM Driver Service Status

  • Open Services.msc and locate “SQL Server FILESTREAM” or check via command line:
  • Run: sc query filestream in an elevated Command Prompt.
  • If the state shows STOPPED or the service is missing, this confirms the driver is not loaded into the kernel.

Step 2: Enable FILESTREAM at the Operating System Level

  • Open SQL Server Configuration Manager.
  • Navigate to SQL Server Services > right-click your instance (e.g., MSSQLSERVER) > Properties > FILESTREAM tab.
  • Check “Enable FILESTREAM for Transact-SQL access” and “Enable FILESTREAM for file I/O streaming access.”
  • Apply changes and restart the SQL Server service.

Step 3: Manually Reload the Kernel Driver

  • Open an elevated Command Prompt and run: sc start filestream
  • If it fails with error 1058 (service disabled), set the start type to automatic: sc config filestream start= demand
  • Reboot the server to ensure the driver initializes before SQL Server starts.

Step 4: Check Group Policy and Security Software Interference

  • Review local or domain Group Policy under Computer Configuration > Administrative Templates > System > Filter Manager for any restrictions on loading minifilter drivers.
  • Temporarily disable third-party antivirus/EDR software to rule out driver-blocking behavior, then retest.

Step 5: Repair or Reinstall SQL Server Database Engine Services

  • If the driver files are missing or corrupted (common after in-place OS upgrades to Windows Server 2025), run the SQL Server 2025 setup media.
  • Choose Repair for the instance, which restores the FILESTREAM kernel driver binaries to C:\Windows\System32\drivers\.

Alternative or Verification Steps

  • Confirm the fix by running sc query filestream again and verifying the state is RUNNING, then execute SELECT * FROM sys.dm_filestream_non_transacted_handles; in SSMS to ensure no errors are thrown.
  • If the issue persists, check the System Event Log for Filter Manager (FltMgr) errors, as a conflicting third-party minifilter driver may need to be temporarily unloaded using fltmc unload <DriverName>.