April Windows updates block kernel driver, breaking third-party backup tools
Microsoft confirms the April 2026 security updates added psmounterex.sys to the Vulnerable Driver Blocklist, which can cause VSS-based backup programs (e.g., Macrium Reflect, Acronis Cyber Protect Cloud, UrBackup Server, NinjaOne Backup) to fail when mounting or managing backup images on Windows 10/11/Server. Symptoms include VSS timeouts and Code Integrity events (Event ID 3077). The company recommends updating affected backup apps to versions that use newer drivers and warns against uninstalling the update. Some Windows Server 2025 devices may experience BitLocker recovery prompts after KB5082063, and Microsoft issued out-of-band fixes for server install/restart issues.