When you tell MO2 to close, it first needs that external application to release the VFS lock. Otherwise, if MO2 shut down while the other app was still writing to the virtual folder, you could end up with corrupted configs, missing plugin data, or a broken VFS cache.

If you find yourself force-closing MO2 repeatedly, run before launching MO2—this sometimes allows better process tracking.

You can also manually check which process has a lock using (Sysinternals): Search for modorganizer.exe → Properties → Handles → look for \Device\VFS . Bottom line: Mod Organizer is trying to protect your setup from corruption. When you see that message, 90% of the time a mod tool is still running in the background. Close it properly, and MO2 will exit instantly.