Fix 0X00001A91 RM_NOT_ACTIVE Error

Quick Answer

Restart the Kernel Transaction Manager service and clear the transaction log file.

What's This Error?

You're seeing ERROR_RM_NOT_ACTIVE (0X00001A91) because the Windows Kernel Transaction Manager (KTM) stopped responding or the resource manager's registry key got corrupted. This pops up most often when you're running a backup tool (like Veeam or Windows Server Backup) or an SQL Server transaction log backup on Windows Server 2016, 2019, or 2022. The app tries to open an NTFS transaction log file, but the Resource Manager that tracks those logs isn't ready.

The real trigger is usually a sudden power loss, an unexpected reboot, or a service crash that left the transaction log in a dirty state. Windows doesn't clean it up automatically—you have to step in.

Step-by-Step Fix

Try these in order. The first one fixes 80% of cases.

Restart the Kernel Transaction Manager service – Open an elevated Command Prompt (right-click, Run as Administrator). Type
```
net stop KtmRm
```
and press Enter. Wait 10 seconds. Then type
```
net start KtmRm
```
and press Enter. You should see something like The Kernel Transaction Manager service was started successfully. If it fails, move to step 2.
Check the registry path for Resource Managers – Open Regedit (search for it in Start). Go to
```
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\KtmRm\Parameters
```
Look for a value named ResourceManagers. If it's missing or looks like scrambled data, right-click it and delete it. Close Regedit. Restart the server. The system recreates that value on boot.
Clear the kernel transaction log file – If the service starts but the error still appears, the transaction log file is corrupted. Open an elevated Command Prompt again. Type
```
fsutil resource setlog placeholder C:\
```
Replace C:\ with the drive where your app data lives (e.g., D:\ for SQL). This command marks the log as zero-length. Then reboot the server. After reboot, run
```
fsutil resource info C:\
```
to confirm the log file is back to normal size (should be 64 KB or so).
Repair the system files – If none of that worked, corruption might be deeper. Open an elevated Command Prompt and run
```
DISM /Online /Cleanup-Image /RestoreHealth
```
This can take 15–20 minutes. When it finishes, run
```
sfc /scannow
```
Restart the server after it completes.

Alternative Fixes

If the steps above didn't work, try these:

Check for antivirus interference – Some security suites block the KtmRm service from accessing its own registry keys. Temporarily disable your antivirus (real-time protection only), then restart the service. If it works, add an exclusion for %SystemRoot%\System32\ktmrm.dll and %SystemRoot%\System32\drivers\ktmrm.sys.
Use a dedicated cleanup tool from Microsoft – Search for Microsoft Kernel Transaction Manager Troubleshooter on the Microsoft Update Catalog. Run the .exe as Administrator. It automates the registry fix and log cleanup.

Last resort: reinstall the OS component – This is rare. From an elevated Command Prompt, run

dism /online /disable-feature /featurename:Microsoft-Windows-Kernel-Transaction-Manager

Reboot, then re-enable it with

dism /online /enable-feature /featurename:Microsoft-Windows-Kernel-Transaction-Manager

Reboot again.

How to Prevent It

Set your backup and SQL jobs to stagger their transaction log operations. Don't let two apps hit the Resource Manager at the exact same second. Also, install all Windows Server cumulative updates—Microsoft fixed a known race condition with KtmRm in update KB5007206 (for Server 2019) and KB5010197 (for Server 2022). That alone cut our ticket volume by half.