Quick answer
Restart the application that shows the error. If that doesn't work, kill and restart Windows Explorer via Task Manager. No reboot needed in most cases.
What's actually happening
This error fires when some code tries to register a window as a drop target using RegisterDragDrop(), but that window (or its thread) already has one registered. It's not a corrupt file or a driver issue — it's a program stepping on itself or another program's toes.
I see this most often after uninstalling a third-party file manager like Total Commander, XYplorer, or Directory Opus. Those tools hook into Explorer's drag-and-drop system, and when they're removed, the registration sometimes gets left in a half-baked state. Also happens when an app crashes after registering a drop target but before unregistering it.
Had a client last month whose entire print queue died because of this — their document management software registered itself as a drop target for the print spooler window, then crashed. Every new print job hit this error and the queue stalled.
Step-by-step fix
- Close the offending application. Not just minimize — fully exit it.
- Open Task Manager (Ctrl+Shift+Esc). Check if the app still has a background process running. If yes, end that process tree.
- If the error came from a file dialog or desktop, restart Windows Explorer:
- In Task Manager, find Windows Explorer in the Processes tab.
- Right-click and select Restart. - Test drag-and-drop again. If the error is gone, you're done.
- If it persists, log off and log back in. This clears all COM registrations for your session.
If the error returns
Some software re-registers at every launch. Find the culprit by checking what started just before the error first appeared. Look at your startup programs (msconfig or Task Manager Startup tab) and disable anything that manages files or clipboard. Common offenders:
- Clipboard managers (Ditto, ClipClip)
- Old shell extensions (check with ShellExView)
- Third-party file managers that hook into Explorer
Boot into Safe Mode. If the error stops, you've got a third-party conflict. Disable startup items one by one until you find it.
Prevention tip
When uninstalling software that hooks into drag-and-drop, reboot immediately after uninstall. Don't just close the installer. The uninstaller often can't clean up active registrations, and a reboot forces the cleanup. Also, avoid running two file managers at the same time — they fight over the same COM registrations and you'll see this error within minutes.
If you're a developer hitting this in your own code: always pair RegisterDragDrop() with RevokeDragDrop() on shutdown, and handle the case where the window is already registered by unregistering first or skipping the call.