[OS X TeX] All TeX files are locked!

[André Bellaïche]

Dear all,

I need help for some other trouble. From this afternoon on, I cannot  
modify a TeX file (TeXShop) any more. When I want to save the file, I  
get a first message (in French) saying:

The file "F.tex" is locked. Do you want to overwrite it anyway?

When i say "yes", I get a second message, saying:

You do not have permission to save "F.tex" in the directory "so and so".

However, I can save a copy under a new name.

What puzzles me is that happens with all TeX files from different  
users on my Imac, but not with Excel, Word or Adobe Illustrator files.

Any idea?

André