The April 2026 KB5083769 security update breaks third-party backup applications from multiple vendors on systems running Windows 11 24H2 and 25H2. According to user reports, first spotted by Microsoft MVP Susan Bradley, this issue affects software using VSS (Volume Shadow Copy Service) snapshots and causes failures due to a VSS service timeout. Microsoft VSS was introduced in Windows Server 2003 and helps ensure that the operating system, backup software, and business apps (such as SQL Server and Exchange) work together more effectively. VSS is also used by Windows features and applications like Windows Server Backup, System Center Data Protection Manager, and System Restore. The list of software impacted by this known issue includes, but is not limited to, products from Acronis (Cyber Protect Cloud), Macrium (Reflect), NinjaOne Backup, and UrBackup Server. Acronis has also published a support document confirming that the issue affects Windows 11 Pro and Home editions, causing backup operations to fail with the error "The backup has failed because Microsoft VSS has timed out during the snapshot creation" after installing the KB5083769 update. "This update may introduce system-wide issues that affect Microsoft VSS (Volume Shadow Copy Service) operations, leading to backup failures," Acronis says. "In some cases, the affected machine may also lose connectivity with the cloud console and appear offline." As a temporary workaround, affected users are advised to uninstall the problematic "Security update for Microsoft Windows (KB5083769)" from Settings > Windows Update > Update history > Related settings > Uninstall updates, then pause Windows updates and reboot the system. BleepingComputer reached out to Microsoft for more information, but a response was not immediately available.