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.

In a new update first spotted by Susan Bradley of AskWoody, Microsoft says its investigation found that devices affected by recent boot errors were previously unable to install the December 2025 security update.

When the failed update was rolled back, Windows was left in an unstable state, or "improper state."

"Recent investigations have determined this issue can occur on devices that failed to install the December 2025 security update and were left in an improper state after rolling back the update," reads the updated advisory.

"Attempting to install Windows updates while in this improper state could result in the device being unable to boot. We are working on a partial resolution that will prevent additional devices from resulting in a no-boot scenario if they try to install an update while in this improper state."

"However, this partial resolution will not prevent devices from getting into the improper state in the first place, nor will it repair devices that are already unable to boot."

The company says it continues to investigate why some devices fail to install Windows updates or end up in this improper state following a rollback.

Microsoft says that the issue is currently limited to physical devices, with no reports of virtual machines being impacted.

BleepingComputer contacted Microsoft with further questions about this issue and will update the story if we receive a reply.