Bizarre behaviour. Couldn't reproduce that last test. Or several others, including outside the Dropbox folder. For example I saved a new file in
C:\Users\terry\TestRename\TestRename.ino
and successfully changed it to
C:\Users\terry\TestRenameChangedToThis\TestRenameChangedToThis.ino
But then I manually changed that to
C:\Users\terry\TestRename2\TestRename2.ino
I then opened it and tried changing it with Rename to TestRename2ToANewName
but that failed, with the error.
Tried that again, renaming to TestRename3, and that failed too.
Tried a minor edit to code and saved file before the attemp to rename, but that failed too.
Same with filenames as simple as A.ino. Only seem to succeed when renaming a new file for the first time.
EDIT: This may be relevant.
After spending couple of hours I have to drop it for now.