Microsoft has released Windows 11 cumulative update KB5083769 in April 2026, which introduces a significant compatibility issue for enterprise backup solutions. The update blocks the psmounterex.sys kernel driver, a component required by several major data protection applications. This action is part of Microsoft's ongoing security hardening efforts to address known vulnerabilities in third-party drivers.
The driver block affects Windows 11 versions 24H2 and 25H2. It also applies to the optional update KB5083631. Backup software including Macrium Reflect, Acronis Cyber Protect Cloud, NinjaOne Backup, and UrBackup Server fail to mount images or create snapshots after the update installs. Users report experiencing VSS timeout errors or VSS_E_BAD_STATE errors during backup operations.
Acronis has confirmed that its Cyber Protect Cloud product is impacted by this change. The company states that affected machines may lose their connection to the cloud console and appear as offline. Macrium Reflect has indicated that an updated driver is currently in development to resolve the incompatibility. The issue is linked to CVE-2026-32202.
Microsoft describes the driver block as a necessary security measure rather than a bug. The company has not confirmed a schedule for an irregular patch to fix the driver issue. Users can currently uninstall KB5083769 to restore backup functionality, but this action removes security patches including fixes for the related vulnerability.
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