Got the same error on a new build with paid licence and different project.
It seems if a virus checker is running while the project is open, it fails when saving - I didn't record the error as the loss of an hours work encouraged me to go pace and make a cup of tea instead.
Are references created somewhere in the temp directory while the project is open , that can be deleted by virus/disk cleaning facilities that then could cause either failure or corruption of the file upon attempted saving?
Though the project was still open in the window, and appeared fine, I got a similar message to the one in the original post - leading me to believe this is similar to what happened last time.