Windows 11 KB5124008 was supposed to be one of the year’s best updates, at least on paper. It restored Windows 10-era features, including a movable taskbar and a resizable Start menu, but unfortunately, it’s causing issues on some PCs.
Windows 11 KB5124008 was released on September 8 as a mandatory patch, and Microsoft says the update breaks WSL-based apps and Remote Desktop sessions. But our investigation found more bugs, including one that breaks File History backups and another that causes Explorer.exe crashes and system/GPU stability issues.
Ironically, all of this comes days after Microsoft warned users not to delay updates due to increasing AI-assisted security threats.
“If you’re not delivering critical quality updates with security fixes until a couple of weeks after they’ve been issued, that’s ample time for attackers using AI to find and exploit known security gaps,” the Microsoft 365 director warned.
“To address this, we’ve updated our recommendations for deploying Windows updates to less than three days as the deferral period for quality updates, setting deadlines for those updates to zero or one day, and the update grace period to a maximum of two days.”
Anthropic says Microsoft broke Claude Cowork on Windows 11, and Microsoft admits it
In updated documentation spotted by Windows Latest, Microsoft confirmed a bug in Windows 11 KB5124008 that breaks apps using HCS-managed Linux virtual machines with Plan9 shared folders.
For example, if you use Claude Cowork and you’ve given it access to your preferred folder or files via its Workspace, that integration is now broken.
As soon as I installed Windows 11 KB5124008 and tried running my regular Claude Cowork session, I noticed that the Cowork workspace would simply not connect and asked me to either reinstall the desktop app or try again. It also told me to run the commands that I wanted Claude Cowork to execute myself.
Turns out I was not alone, as Windows users are flagging issues with Claude Cowork, and there’s also a bug report on GitHub.
In our tests, we found that Windows 11’s September update changed the Plan9/9P filesystem-sharing implementation used around Hyper-V, and that new implementation rejects Cowork’s previously valid token-backed Tattach request with EINVAL.
This means Windows can still create the virtual machine that allows Cowork to run, and it even attempts to give it access to the file system, but the Linux guest can no longer actually attach to and mount it.
In an updated changelog, Anthropic confirmed it’s a bug caused by the Windows update, and it doesn’t know how to patch it because it’s broken by Microsoft. But for now, if you’ve installed the latest Claude desktop update, it’ll correctly explain why Cowork couldn’t connect to your drive.
“Claude couldn’t connect your C: drive to its workspace, so it can’t access your local files. Reinstalling won’t fix this. Your chats and files are safe. A Windows update released on September 8 prevents Claude’s workspace from reaching your files. We’re tracking this issue. Claude Code is unaffected,” the error reads, and here’s what happens when you still force run it:
Microsoft describes the same underlying problem in its documentation:
“After installing the September 2026 security update KB5124008, applications that use HCS-managed virtual machines might experience issues when sharing host folder with Linux VMs using Plan9,” Microsoft noted. “Standard Hyper-V virtual machines that do not use the Plan9 feature are not affected by this issue.”
Microsoft says it’s working on a solution, but has nothing to share for now.
Microsoft says Remote Desktop Services (RDS) is broken
There’s another bug in Windows 11 KB5124008 where some of you can no longer use Remote Desktop Services (RDS).
This specifically affects enterprises or anybody who uses Remote Desktop on Windows, and it’s not the first time Microsoft has broken this feature. If you install the update, you might observe that RDS is unstable and connections fail frequently, to the extent that it’s impossible to maintain remote connections.
There are also sign-in issues and another bug where you’d be stuck on a “Please wait for the Remote Desktop Configuration” dialog. And if you manage to get through, you may run into other problems, including unstable or unresponsive File Explorer and a broken Microsoft Management Console (MMC).
“Additionally, the Windows Update page might stop responding and continuously display a loading indicator,” Microsoft warns in another update to its documentation spotted by Windows Latest.
In case you run into problems with Remote Desktop, Microsoft says you should reach out to the company through support during weekdays or wait for the next Windows update, which might fix the botched RDS experience.
Microsoft appears to be investigating Explorer.exe crashes (enterprises are mostly affected)
There’s a potentially serious bug in Windows 11 KB5124008 that causes some PCs to show a black screen after you sign in because Explorer.exe either doesn’t start correctly or crashes almost immediately.
Explorer.exe goes beyond just File Explorer, as it powers your desktop, taskbar, and even the Start menu, so when it’s broken or crashing, you can’t do much except restart the process in Task Manager.
In a Reddit post, Jason Parker, who works for Microsoft, confirmed the company is looking into the reports and asked for memory dumps to investigate the root cause.
Based on my early analysis, Explorer.exe issues seem to particularly affect PCs with certain virtual desktop environments where user profiles are stored in containers. So far, I’ve seen reports mostly coming from those who use Citrix UPM, FSLogix, Horizon, and ProfileUnity ProfileDisks, and both Windows 11 24H2 and 25H2 are affected.
Parker noted that Microsoft still needs more data to determine the root cause.
“The September Windows updates were extensive and need more data to understand and root cause,” Microsoft noted.
That explains why the September 2026 Update is a mess on some PCs. It’s an extensive release, which means it changed hundreds or thousands of files, and Microsoft can’t possibly figure out all the issues.
For now, you should try restarting Explorer manually or creating a fresh profile if explorer.exe crashes.
File History backup feature is broken
Microsoft hasn’t confirmed it yet, but I’ve seen multiple reports pointing to a regression in the File History feature in Windows 11.
Multiple users told Windows Latest that File History is entirely broken because it simply cannot detect any external backup drive or hardware. It means that even if you’ve connected a drive, for example, and try creating a backup, File History fails and says you have not connected the drive.
For those unaware, File History is a built-in Windows backup feature that automatically creates copies of your files and maintains older versions. It has a specific use case, and Microsoft hasn’t really bothered to improve File History in all these years, but it was working well until the Windows 11 KB5124008 update.
Beyond the reports I’ve personally received, I’m seeing similar complaints on Reddit and Microsoft’s forums, with one user stating that they got in touch with Microsoft support, which suspects the recent update is indeed responsible.
Right now, it’s unclear what needs to be done to restore File History functionality, as we’ve yet to hear from Microsoft, which is most likely aware of the reports. While users have confirmed that removing Windows 11 KB5124008 allows File History to work again, I don’t recommend going that route because it’s a critical security update.
Windows 11 KB5124008 is causing issues on some AMD PCs
We’re also seeing reports where Windows 11 KB5124008 appears to be causing serious GPU issues on AMD Radeon PCs. In a thread on Reddit, users report AMD driver timeouts, black screens, complete system freezes, missing displays, and in some cases, the GPU driver appears to break again immediately after it is reinstalled.
It’s unclear why only some AMD PCs are running into problems, but reports point to multiple Radeon GPUs, including RX 6600, RX 7700 XT, RX 7800 XT, RX 7900 GRE, RX 7900 XTX, and even RX 9070 XT systems.
Removing the September update appears to resolve the issues for affected users, which makes Windows Update the stronger suspect right now, although it’s still too early to say exactly what is broken.
And that’s all for now. We’ll likely learn more from Microsoft in the coming days, but if you’re having issues and considering removing Windows 11 KB5124008, or skipping it if you haven’t installed it yet, I’d recommend against that move unless you’re actually affected by a critical problem. This is a critical update that patched hundreds of security issues.
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