Microsoft confirms April Windows updates cause backup failures

Microsoft's April 2026 security update added the kernel driver psmounterex.sys to its Vulnerable Driver Blocklist to mitigate CVE-2023-43896, a high-severity buffer overflow; as a result, backup products that rely on that driver (including Macrium Reflect, Acronis Cyber Protect Cloud, UrBackup Server, and NinjaOne Backup) may fail to mount or restore image backups on Windows 10/11 and Windows Server systems, with VSS timeouts and Code Integrity (Event ID 3077) entries observed — Microsoft advises installing updated application versions whose drivers include the required protections rather than uninstalling the security update.