I got this issue just now as well, and it turned out that I had a file open in another window that I had forgotten about...
Yes, makes sense. The folder I use is not in OneDrive, because I had problems using folders for which I use SVN with OneDrive before. And the problem started after I switched to 9.12.1 last Friday.
Try steps below.
- App->Clean Deployment Directory
- Run locally
I have came across the same error when creating a widget and working on it on VS Code.
It got solved when I closed the terminal where i had run 'npm run start' for the widget.
So I suspect another program is hijacking the folder where the deployment files need to be modified when you run your project.