Microsoft is urgently investigating serious stability issues triggered by the January 13, 2026, security update for Windows 11, which is causing some PCs to fail to start and become unusable.
The problematic update, KB5074109, was released for Windows 11 versions 25H2 and 24H2 to deliver security fixes.
Instead, many users report that their systems crash with an “UNMOUNTABLE_BOOT_VOLUME” error and refuse to boot normally.
Boot Loops and Black Screens After January Patch
After installing the January Patch Tuesday update, some devices enter a boot loop or show a Black Screen of Death immediately on startup, as reported by CSN.
Affected users see stop code 0xED (UNMOUNTABLE_BOOT_VOLUME) with the message:
“Your device ran into a problem and needs a restart.”Microsoft has confirmed it is aware of the issue and says it has received a “limited number of reports” of devices that cannot complete startup.
So far, the problem appears to affect only physical hardware. In contrast, virtual machines are not affected, as first highlighted by AskWoody.
In many cases, users must enter the Windows Recovery Environment (WinRE) and manually uninstall the update to regain access to the system.
Even when systems do boot, KB5074109 is linked to wider instability. Users report:
- Full system lockups
- Graphics driver conflicts
- Black screens where the Windows desktop never appears
GPU-heavy tools such as BforArtists 5.0 (a Blender fork) can cause the system to freeze when switching viewport shading modes, without showing a blue screen.
These “silent” hangs are believed to stem from conflicts between new kernel changes and existing DirectX/GPU drivers on both Nvidia and AMD hardware.
The update also disrupted enterprise environments. Microsoft confirmed it broke credential prompts for Azure Virtual Desktop (AVD) and Windows 365, blocking remote connections until an out-of-band fix, KB5077744, was shipped on January 17.
Some Outlook Classic users also report freezes on launch and failed POP/SMTP sync due to stuck background processes.
Mitigation and Safety Advice
For systems hit by UNMOUNTABLE_BOOT_VOLUME, the current workaround is:
- Boot into WinRE
- Uninstall the latest quality update (KB5074109)
For unstable but bootable systems, users and admins are advised to:
- Pause Windows Updates, or
- Remove KB5074109 via:
- Settings > Windows Update > Update history > Uninstall updates
Enterprise administrators should deploy Known Issue Rollback (KIR) policies where available.
Home and business users are strongly encouraged to delay installing KB5074109 on Windows 11 25H2 until Microsoft publishes a complete fix.
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