April Windows 11 security uopodate breaks third party backup apps.
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Updated May 2: This article about the Windows KB5083769 security update breaking third-party backup applications has been updated to include a statement from Microsoft.
Are you using Windows 11 versions 24H2 or 25H2? If so, you might want to not download April’s KB5083769 cumulative security update after multiple reports of third-party backup applications breaking following installation. At the time of writing, the backup vendors known to be impacted by this security update failure include Acronis, Macrium, NinjaOne and UrBackup. Here’s what we know so far and the mitigation steps to take until Microsoft resolves the issue.
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While it is always good practise to install security updates as soon as possible, sometimes there are unforeseen consequences. Unfortunately, Microsoft has something of a track record in the update failure stakes, from disappearing passwords to broken VPN connections. The latest security update issue stems from the installation of the April cumulative Windows 11 security update, reference KB5083769, which has now been reported as causing backup applications from multiple vendors to fail due to a service timeout issue.
The broken backups issue first came to light on April 29, when it was reported that the Windows 11 24H2 and 25H2 April update was causing problems for users across support forums, official and otherwise. Although, at the time of publication, the official Microsoft support pages for KB5083769 shows no details of known issues with backups, the problem is thought to sit with the Volume Shadow Copy Service. Software using these VSS snapshots are seemingly experiencing a service timeout which, in turn, cause the backups to fail.
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I reached out to Microsoft for a statement and a spokesperson proviued the following: “In the April 2026 Windows security update, we added known vulnerable kernel driver psmounterex.sys to the Vulnerable Driver Blocklist. Backup applications that rely on this driver may experience failures when attempting to mount or manage disk images. We do not recommend uninstalling or pausing this update. Customers with an impacted driver should install the latest application versions and validate against the driver blocklist to remain protected. For more information, please see here.”
However, in the meantime, if your backups are failing then seek support from your vendor in the first instance to determine if the cause is likely to be the Windows 11 security update. If it is, then Acronis has advised users, by way of a temporary workaround, to uninstall the KB5083769 update and reboot the machine. “After removing the update and restarting the system,” Acronis said, “backup operations should resume successfully without requiring any changes to Microsoft VSS or the Acronis agent.”
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