Okay, I think I've figured out what caused mine, at least.
I went and did several of the things I did earlier today that I don't usually and managed to duplicate the error and then use
Resource Monitor to ID the other process. Which was explore.exe--the basic Windows File Explorer.
And one of the unusual things I had done was run a file search in File Explorer.
Closing all the File Explorer windows (I'd left the search some time ago) closed the process after a short delay and I could rename that book again without error.
I knew that an open File Explorer window into a dir could cause issues, but not that a search done hours ago could...
