CO_E_APPSINGLEUSE (0X800401F6) — App already running

Quick answer for pros: Kill the hung process in Task Manager, or delete the registry key under HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\SessionInfo\1\ApplicationViewManagement for the broken app. No reboot needed.

Why this happens

The CO_E_APPSINGLEUSE error (0X800401F6) fires when a program uses COM single-instance mode and it detects another copy already running — but the lock is stale. This is common with older or poorly-coded apps that crash without releasing their COM object. I see it most with:

• Visual Studio 2015-2019 when a solution crashes hard
• Some Adobe Creative Suite installers (especially after a failed update)
• Custom in-house tools that use CoRegisterClassObject with CLSCTX_LOCAL_SERVER

The real trigger is the app's process or a leftover COM stub that didn't get cleaned up. The registry key mentioned above holds a list of active single-instance apps for the current user session. If that key gets corrupted or orphaned, you get the error.

Fix steps (main method)

1. Kill the stuck process. Open Task Manager (Ctrl+Shift+Esc). Look for the app in the Processes tab. If you see it, right-click and End task. If you don't see it, go to Details tab, sort by name, and kill any leftover .exe for that app. Sometimes it hides as a background process — check there too.
2. Clear the ApplicationViewManagement registry key. Press Win+R, type regedit, hit Enter. Navigate to:
   HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\SessionInfo\1\ApplicationViewManagement
   Look for a subkey with the app's name or a GUID. Right-click that subkey and Delete. Close Regedit.
3. Restart the app. If it still fails, reboot. Nine times out of ten, this combo fixes it.

Alternative fixes if the main one fails

If the above doesn't work, try these in order:

1. Run the program as administrator. Right-click the shortcut and select Run as administrator. Some single-instance locks are per-user, and admin privileges can override a stale lock.
2. Disable COM single-instance for that app. This is a hacky one. Go to the app's install folder, rename the .exe to something like appname_original.exe, then create a batch file that launches a new instance every time. Not pretty, but works for in-house tools where you have control.
3. Use Process Explorer (Sysinternals). Download it from Microsoft. Search for handles named after your app — sometimes a handle leak keeps the lock alive. Close those handles.
4. Last resort: System Restore. If the error started after a Windows update or software install, roll back to a restore point from before that change.

Prevention tip

Stop the app properly. Don't force-close it with Task Manager unless you have to. Most single-instance apps release their COM lock on normal exit. If you crash one frequently, check for a newer version or a config flag like /nosingleinstance — some apps support it. Also, keep Windows and your antivirus updated; I've seen security software block COM registration and cause this.

Bottom line: kill the process, blow away the registry key, move on. If it keeps happening, the app is the problem, not Windows.