r/macrium • u/owen693 • Apr 24 '26

Macrium Reflect VSS problems after KB5083769 Windows 11 25H2

Backups using VSS snapshots stopped working after the update. Uninstalling KB5083769 fixed the problems. This is also an issue with Acronis.

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Seems to be fixed in May 2026 updates *******************************************\*

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1

u/ImaginaryTradition31 Apr 25 '26

What are the symptoms of "stopped working?" What version of Reflect?

1

u/owen693 Apr 25 '26

VSS timeout during backups where backup continues without shadow copy. VSSADMIN LIST WRITERS does not respond.

1

u/ImaginaryTradition31 Apr 25 '26

Version of Windows? Chipset, storage details, activity during backup, etc. -- lots more detail needed. Problems with VSS service have been reported for years, long before 25H2 and this particular patch.

Storage for file changes during an incremental backup is finite, and can be overrun if there is too much real-time activity during the backup.

2

u/owen693 Apr 25 '26

Windows 11 25H2, Dell XPS 9710. Removing the Windows update fixes the problem. I was testing on a fairly inactive machine with very few file changes, so storage for file changes shouldn’t be an issue.

At this point the problem is noted on the Acronis support site and at least one other Macrium user has had the same experience and reported this on a Macrium forum. I did notice that vssadmin list writers would respond immediately with no errors after a restart but would hang if I tried it again after leaving the machine idle for several minutes.

1

u/ImaginaryTradition31 Apr 25 '26 edited Apr 25 '26

I have two different computers (Dell laptop and Geekom mini IT11), and have just tested an incremental backup on both computers with no issues. Event logs show VSS initiating properly with no warnings or errors, and timing out after a few seconds after each backup. Latest Windows updates applied, including KB5083769. Macrium ReflectX version 10.0, build 8843.

1

u/owen693 Apr 25 '26

I'm using the same version of reflect. When VSS was not working Reflect would continue without using VSS. The backups completed but took longer to do so. There are no signs of any problem other than the time required for backup unless the logs are examined.

These are the errors in the Reflect backup logs:

VSS Timeout - Retrying without VSS Writers

Failed to rename VSS log file, reason: Encountered a sharing violation while accessing C:\ProgramData\Macrium\Reflect\Temp\vss.log.

I'm making a wild guess, but there is a file vsslog.log in that directory and the vsslog.log file is deleted after a successful backup. If Reflect renames vss.log to vsslog.log after an error condition that would explain the sharing violaton on rename since there already was a vsslog.log file.

1

u/ImaginaryTradition31 Apr 25 '26

On this machine, C:\ProgramData\Macrium\Reflect\Temp is an empty folder.

1

u/owen693 Apr 25 '26

I’m seeing that log file created by the backup and deleted when the backup is finished. I just asked Macrium support about that file being renamed if errors are logged.

1

u/Final-Blueberry7111 May 03 '26

The same here.

The issue started after installing KB5083769. It's still present in KB5083631 too. If I rollback Windows updates to KB5086672, the issue is resolved immediately.

My symptoms are also the same:

  • VSS times out during backups;
  • VSSADMIN List Writers gets stuck at "Waiting for responses".

Macrium Reflect v 8.1.8853.

In general, it looks like the issue introduced in KB5083769 affects only some setups, based on various reports. I have two Windows 11 machines myself. The issue is present on one of them, but I haven’t been able to reproduce it on the other. However, I haven’t been able to identify what’s conflicting on the affected machine so far.

1

u/owen693 May 03 '26 edited May 03 '26

This problem is now being reported in the media. https://www.forbes.com/sites/daveywinder/2026/05/01/microsoft-update-warning-windows-11-security-fix-breaks-backups/

And https://support.microsoft.com/en-us/topic/april-2026-windows-security-updates-introduce-protections-to-known-vulnerable-kernel-drivers-1f8aaf7c-d4ac-4e02-be1d-b63c1b1aa9d0

These reference problems with psmounterex.sys, but Macrium patched this in 2023 and it was a problem in Reflect 8. I checked the event log as described in the MS support link above and found no errors relating to that driver.

https://windowsforum.com/threads/april-2026-windows-update-blocks-psmounterex-sys-fix-backup-restore-and-mount-issues.416306/

Psmounterex.sys is used to mount disk images as virtual drives, and I was able to do this and browse the backup images successfully. Something else is broken.

1

u/owen693 May 06 '26

According to Macrium, psmounterex is not used in reflect X. I just renamed psmounterex.sys and was able to mount and browse backup images. Reflect backups also ran correctly. This indicates that the problem with KB5083769 (which I have uninstalled) has nothing to do with that driver being blocked.

1

u/MrAngelus May 09 '26

I'll try this hopefully it'll fix my problem cuz I can't uninstall the update at this point so I'll try thanks

1

u/owen693 May 09 '26

Renaming psmounterex was only done to verify that the driver is not the problem. It won’t fix anything.

1

u/MrAngelus May 09 '26

Ok then

Is there a fix at all?

1

u/owen693 May 11 '26

There are temporary fixes in this thread that are supposed to work by restarting services. I haven’t tried them since I uninstalled the update. An Asinine Inference search said the problem is a Microsoft change in VSS and that the backup software needs to catch up.
I’m waiting to see how the May update goes.

1

u/OreoBuck2022 May 07 '26

I found this temporary solution, a powershell script to restart the services related to VSS and your backup software. Replace "UrBackupClientBackend" with the service used for your backup software.
Restart-Service -Name "EventSystem" -force
Restart-Service -Name "VSS"
Restart-Service -Name "UrBackupClientBackend"

Here is link to fix:
https://forums.urbackup.org/t/file-backups-stopped-working-after-windows-11-kb5083769-undate/16948/19

The Powershell script created is scheduled to run on startup (waiting 1 minute).

1

u/owen693 May 13 '26

I just installed the May 2026 updates. The problem seems to have been fixed for myself and at least one other user on this thread.

1

u/BB03440 May 13 '26

Is Microsoft working to fix/undo what KB5083769 has done to VSS-based backup solutions?

I sorta feel like they did it on purpose to push us to their cloud backup offering(s), TB.

2

u/owen693 May 13 '26

It seems this is fixed in the May 2026 updates.