File xxx has been modified by another user

Hello

I am struggling with an issue I can't get rid of. When using one of my InDesign (CS 5.5) document, everything will go ok during a few minutes and then, when I try to save it, the following message appears : "The file xxx has been modified by another user. InDesign will close to prevent any damage" (this is actually a translation, I am French and the original message is in that language). I have then no choice than CTRL - ALT - DEL, close InDesign and start it again. I work on a local PC, with no network and no such other user. I tried to change the name of my file, work on a copy, reset preferences, etc., the issue is still there.

Until now, I did not lost any data, and the file is recovered when starting InDesign again, but it is a bit frustrating (it happens realy often) and worrying.

If someone has an idea, I would be very grateful...

Thanks in advance

[Mod.: Added title]