why can't i save my form after changes?

After editing a form I can't save the form to the same filename. I receive the following message:

Needless to say, the form is not shared or open anywhere else.

I've set permissions on the file, folder and designer, I've set compatibility on the urls,  but nothing makes a difference. Imagine saving every few minutes and having to give a different file name every time. I have to keep on deleting previous versions.

Any ideas? I'm quite desperate....