Table of Contents
- Introduction
- The Irony: Warn Users to Update, Then Break Everything
- Claude Cowork Broken by Plan9 Regression
- Remote Desktop Services (RDS) Broken
- Explorer.exe Crashes and Black Screens
- File History Backup Feature Broken
- AMD GPU Driver Issues and System Freezes
- Summary of Known Issues
- What to Do If You're Affected
- Final Thoughts
Introduction
Windows 11 KB5124008 was supposed to be one of the year's best updates. On paper, it restored Windows 10-era features — including a movable taskbar and a resizable Start menu. But for many users, it's turned into a buggy nightmare that breaks multiple core features.
Released on September 8, 2026 as a mandatory patch, KB5124008 has been confirmed by Microsoft to break WSL-based apps, Remote Desktop sessions, and more — with additional bugs discovered by security researchers and users.
If you see Windows 11 Build 26200.9445 in Settings → System → About, you have the September 2026 Update that contains these issues.
The Irony: Warn Users to Update, Then Break Everything
The timing couldn't be more awkward. Days before releasing KB5124008, 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." — Microsoft 365 Director
Microsoft even updated its recommendations to require deployment within less than three days, setting deadlines to zero or one day, and a maximum grace period of two days.
Then KB5124008 broke features across the operating system.
Claude Cowork Broken by Plan9 Regression
One of the most significant confirmed issues is a bug that breaks apps using HCS-managed Linux virtual machines with Plan9 shared folders.
Confirmed by both Microsoft and Anthropic:
- Microsoft documented the issue in its release health dashboard
- Anthropic confirmed Claude Cowork is affected
- When you install KB5124008, Cowork's workspace can't connect to your C: drive
- Users see error messages asking them to reinstall the desktop app or run commands manually
"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." — Anthropic Error Message
The technical root cause:
- Windows 11's September update changed the Plan9/9P filesystem-sharing implementation used around Hyper-V
- The new implementation rejects Cowork's previously valid token-backed Tattach request with EINVAL
- Windows can still create the virtual machine that allows Cowork to run
- Windows even attempts to give it access to the file system
- But the Linux guest can no longer actually attach to and mount it
Microsoft's documentation confirms: "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. 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.
Remote Desktop Services (RDS) Broken
Another confirmed bug in KB5124008 affects Remote Desktop Services (RDS) — particularly for enterprises and anyone who uses Remote Desktop on Windows.
Symptoms reported:
- RDS instability — connections fail frequently
- Unable to maintain remote connections — connections drop repeatedly
- Sign-in issues — unable to authenticate
- Stuck on "Please wait for the Remote Desktop Configuration" dialog
- Unstable or unresponsive File Explorer
- Broken Microsoft Management Console (MMC)
- Windows Update page stops responding — displays loading indicator continuously
Microsoft recommends reaching out to support during weekdays or waiting for the next Windows update if you run into RDS problems.
Explorer.exe Crashes and Black Screens
A potentially serious bug in KB5124008 causes some PCs to show a black screen after sign-in because Explorer.exe either doesn't start correctly or crashes almost immediately.
Why this is critical:
- Explorer.exe powers your desktop, taskbar, and Start menu
- When it's broken or crashing, you can't do much except restart the process in Task Manager
- This affects both Windows 11 24H2 and 25H2
Who's most affected:
- PCs with certain virtual desktop environments where user profiles are stored in containers
- Citrix UPM users
- FSLogix users
- Horizon users
- ProfileUnity ProfileDisks users
Microsoft's Jason Parker confirmed the company is looking into reports and asked for memory dumps to investigate the root cause.
File History Backup Feature Broken
Microsoft hasn't officially confirmed this one yet, but multiple reports point to a regression in the File History feature in Windows 11.
The issue:
- File History is entirely broken
- It cannot detect any external backup drive or hardware
- Even if you've connected a drive and try creating a backup, File History fails
- It says you have not connected the drive
While removing KB5124008 allows File History to work again, this is NOT recommended because it's a critical security update that patched hundreds of security issues.
AMD GPU Driver Issues and System Freezes
Reports are also surfacing that KB5124008 is causing serious GPU issues on AMD Radeon PCs.
Symptoms reported:
- AMD driver timeouts
- Black screens
- Complete system freezes
- Missing displays
- GPU driver breaks again immediately after reinstalling
Affected GPUs include:
- RX 6600
- RX 7700 XT
- RX 7800 XT
- RX 7900 GRE
- RX 7900 XTX
- RX 9070 XT systems
Removing the September update appears to resolve the issues for affected users, but it's still too early to say exactly what is broken.
Summary of Known Issues
| Issue | Status | Affected |
|---|---|---|
| Claude Cowork / WSL Plan9 folder sharing | Confirmed by Microsoft + Anthropic | Windows 11 24H2, 25H2 |
| Remote Desktop Services (RDS) | Confirmed by Microsoft | Enterprises, RDP users |
| Explorer.exe crashes / Black screens | Investigating by Microsoft | Citrix UPM, FSLogix, Horizon, ProfileUnity |
| File History backup broken | User reports, unconfirmed | Windows 11 24H2, 25H2 |
| AMD GPU driver issues | User reports | Multiple Radeon GPUs |
What to Do If You're Affected
Microsoft is still investigating these issues, but here are practical steps:
1. Don't Uninstall the Security Update
- KB5124008 is a critical security update that patched hundreds of vulnerabilities
- Removing it re-exposes your system to known threats
- Only consider rollback if you're experiencing critical, business-blocking problems — and consult your IT team first
2. Use Workarounds Where Available
- For Claude Cowork: Wait for Microsoft's fix; chat and file reading still work
- For Explorer.exe crashes: Restart Explorer manually in Task Manager or create a fresh profile
- For File History: Use alternative backup solutions temporarily
- For AMD GPU issues: Monitor for driver updates from AMD
- For RDS issues: Contact Microsoft support or wait for the next update
3. Check Official Sources
- Microsoft Windows Release Health — Official issue tracking
- Anthropic Claude Status — For Cowork-specific updates
- AMD Driver Updates — For GPU-related fixes
4. For IT Administrators
- Review your deployment strategy — Consider phased rollouts even for critical updates
- Monitor Microsoft Release Health — Before deploying to all users
- Maintain rollback capability — Have a tested plan for reverting updates if needed
- Document incidents — Track which users are affected by which bugs
- Communicate with users — Set expectations about known issues and timelines
Microsoft is actively investigating these issues and will likely release fixes in the coming weeks.
Final Thoughts
The Windows 11 KB5124008 update is a painful reminder that even critical security updates can introduce significant regressions. While Microsoft was right to warn users about AI-assisted threats and the importance of timely patching, the company's QA process clearly missed several issues that are now affecting users and enterprises worldwide.
Key takeaways:
- KB5124008 — Released September 8, 2026, mandatory security update
- Claude Cowork broken — Confirmed by Microsoft and Anthropic
- Remote Desktop Services — Confirmed broken by Microsoft
- Explorer.exe crashes — Under investigation
- File History — User reports of broken backup functionality
- AMD GPU issues — User reports of driver timeouts and freezes
- Don't uninstall — Critical security update, wait for official fixes
- Monitor official sources — Microsoft Release Health and vendor status pages
If your organization is affected by these Windows 11 issues and needs assistance with patch management, incident response, or IT support, contact SystemLogicIT for expert guidance across the United States and United Kingdom.