Have seen this for years, maybe since the U$R files were added to the program? It seems to usually be related to having a file open for long periods of time, and maybe not working with the file for a while, like opening the file but then leaving the office for an hour and then coming back and working on it. I never was able to nail down exactly what causes it so these are just guesses. We don't see it too often maybe 5-10 times a year per person in our office. It seems to loose track of the .U$R file or something along those lines. Most of the time no one is actually in the file, so we usually just do a save as to a new file name like adding an x to the name and then immediately using Save As again to save back over the original file and it always works. Sometimes another user is actually allowed access to the file while we were in the drawing. In that case the Save As back to the original file is not allowed as the other users access to the .U$R file has replaced our .U$R file with one of their own and we have to hunt down the user who is actually in the file now. I usually locate the actual .U$R file and check it's properties on the Details tab to see who is now in the file. Obviously this last half would not pertain to your situation abanov since the files are all local to your system, but others in networked situations might use the info.
. I am curious though, is the file on a local drive or a networked location?