I opened a project file using Project Professional and the file opened without the "check out" box in the yellow line at the top of the file. This implied that the file had opened automatically in check out mode. This was not the case as I wasn't able to save the file. It gave an error message to the effect that the file was opened in read-only mode and asked if I wanted to save a copy.
I tried to do a 'clear cache' in MS project but there were no files in the cache. The queue status in the job queue showed "Project Save from Project Professional" and the status was "cancelled". I repeated this four times and nothing changed.
The only way that I was able to resolve it was to RUN %appdata% and to delete the cache folder.
Any ideas why this happened?