backup failures

Backup failures on Windows 11 24H2 and 25H2 have been reported following the April 14, 2026 cumulative update KB5083769. The update blocks the psmounterex.sys kernel driver and causes Volume Shadow Copy Service timeouts, breaking third-party backup tools such as Macrium Reflect, Acronis Cyber Protect Cloud, NinjaOne Backup, and UrBackup Server. Microsoft states the driver block is a security hardening measure, not an accident. Users and vendors have linked the failures to VSS snapshot timeouts. The recommended workaround is to uninstall the update where business risk justifies it, pause redeployment, and maintain a clean recovery path until Microsoft or backup vendors provide a permanent fix.