Windows 11 update chaos: Microsoft scrambles as security patch misbehaves

Microsoft is grappling with yet another Patch Tuesday disaster, as the January 2026 Windows 11 security update has left some users unable to boot their PCs and rendered cloud-backed applications, including Outlook, inoperable.

The company confirmed in an online bulletin that a “limited number” of devices are failing to start after installing the January 13 update, showing a black screen with the error UNMOUNTABLE_BOOT_VOLUME.

Affected users must manually recover their systems through the Windows Recovery Environment to uninstall the problematic patch. Microsoft did not provide details on what is causing the boot failures or how widespread the issue is, but experts warn that PCs running Windows 11 versions 24H2 and 25H2 on physical machines are most at risk.

The boot problem adds to a series of issues that have plagued Windows 11 users since this month’s updates. Earlier, PCs running version 23H2 were unable to shut down or hibernate, and versions 24H2 and 25H2 experienced Remote Desktop login failures. Cloud-based apps, including Outlook, OneDrive, and Dropbox, were also impacted – users reported frozen apps, lost emails, and unsaved work.

For Outlook users, the problem was particularly severe. Microsoft acknowledged that classic profiles with POP accounts and PST files stored in OneDrive would hang after the update, sometimes requiring a full system restart. Millions of users were left unable to access emails, with sent items disappearing or re-downloading unexpectedly.

In response, Microsoft has issued two emergency out-of-band updates this month and confirmed a third may be required to fix the boot issue. The company says the updates are cumulative, including all security fixes from the January 13 release and subsequent emergency patches. Users who have already installed the latest updates without issues are not required to act further.

The ongoing problems have sparked criticism from users and experts, who say Microsoft must reassess its update testing procedures. “The quality bar for Windows updates may be at an all-time low,” said one analyst, citing repeated showstopping bugs.

As millions of users await a stable fix, the January 2026 Patch Tuesday rollout stands out as one of Microsoft’s most disruptive updates in recent years, affecting everything from basic system booting to core productivity tools.