Microsoft's September 2026 Windows 11 cumulative update, KB5124008, is generating a growing pile of user complaints in the days since its September 8 release — broken File History backups, a black desktop background bug, install failures with error 0x80240069, and reports of PCs that won't boot after installing it. If your Windows 11 machine has started acting up this week, the update is a likely suspect.

What KB5124008 is supposed to do

KB5124008 is the mandatory cumulative security update for Windows 11 versions 24H2 and 25H2, part of Microsoft's September Patch Tuesday. Microsoft has started rolling out the September 2026 Patch Tuesday (KB5124008 – OS build 26200.9445 and 26100.9445) update for Windows 11. This month's update is particularly important for security-conscious users because Microsoft has addressed hundreds of vulnerabilities, including two security flaws that are already being exploited in the wild.

Because the update is cumulative, PCs that skipped August's optional preview receive that preview's contents bundled in — which is where several of this month's problems originate.

File History backups stop working

Users on Microsoft's own community forums are reporting that File History stopped working after installing the September 8, 2026 updates including KB5124008, KB5126146 (.NET Framework), KB5122878 for Windows 10, and KB890830. These worked normally before the update but stopped working afterward.

One user confirmed the cause by reinstalling and removing the patch: removing KB5124008 resolved it. If you rely on File History as your backup — and many home users do — a silent backup failure is a serious problem, because you won't know your backups are broken until you actually need to recover a lost file.

Black desktop, mouse settings, and boot problems

Microsoft has acknowledged two personalization bugs that carry over from the August preview into the September release. The first affects desktop personalization: some Windows 11 24H2 and 25H2 devices can lose or fail to load desktop settings, leaving the desktop with a solid black background. Slideshow settings and contrast themes may also be affected, and manually restoring the affected settings does not solve the problem because the underlying settings fail to load regardless of the chosen value. Microsoft also says mouse cursor and cursor-animation selections can revert to default settings on non-English Windows installations.

More serious are boot-related reports on a subset of PCs. Some systems enter recovery mode immediately after installing KB5124008, while others get stuck in an endless reboot loop or display error 0xc0430001 during startup. A few users have also reported Secure Boot-related errors that prevent Windows from loading properly. The problem doesn't affect every Windows 11 PC, but reports are increasing, especially from users with HP laptops. These are exactly the symptoms that send a laptop into our walk-in repair bench.

Install failures and rollback loops

For users whose PCs won't take the update at all, error 0x80240069 is showing up frequently. Some users report that Windows 11 KB5124008 fails to install, often with error code 0x80240069 or the generic message "There were some problems installing updates." In other cases, the update reaches a certain percentage, then shows "Undoing changes made to your computer" and rolls back to the previous build. Error 0x80240069 usually indicates a problem downloading or installing the update, often related to network issues, damaged update files, or Windows Update components not working correctly.

Standard remediation is to run the Windows Update Troubleshooter, clear the update cache, or grab the standalone package from the Microsoft Update Catalog. If the PC repeatedly rolls back or ends up in a BitLocker recovery loop, stop and get hands-on help before making it worse — a botched recovery attempt is one of the fastest routes to needing professional PC repair.

What This Means for York, PA

If you're in York County and your Windows 11 PC started showing a black desktop, failing to boot, or asking for a BitLocker recovery key after last week's update, bring it to York Computer Repair at 2069 Carlisle Rd — don't keep rebooting it, and don't wipe it. Call 717-739-9675 first if File History was your only backup, because we may be able to preserve your data before any repair work begins.

Windows 11 September Update KB5124008 Breaks File History, Triggers Black Screens on Some PCs

What KB5124008 is supposed to do

File History backups stop working

Black desktop, mouse settings, and boot problems

Install failures and rollback loops

What This Means for York, PA

Sources

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