Microsoft has linked recent reports of Windows 11 boot failures after installing the January 2026 updates to previously failed attempts to install the December 2025 security update, which left systems in an “improper state.”

The boot failures were first reported earlier this month after users installed the January 2026 Patch Tuesday cumulative update, KB5074109, on Windows 11 versions 25H2 and 24H2.

After installing the update, impacted systems failed to start and displayed a BSOD crash screen with a stop error of “UNMOUNTABLE_BOOT_VOLUME” code.