Editing css resets theme to default

I updated wp from 2.1.1 to 2.1.3 not long ago and after that I’m having an annoying problem.
Whenever I edit the style.css for my template and save it, either from dreaweaver, or an ftp client’s editor or directly in wp, the theme reverts to default.
It doesn’t happen on every save but 8 out of 10.
Never had this problem before, style.css chmod 777 when editing and no new plugins installed.

I tested it with different themes, no difference, by the way, I use a modified version of the theme benevolence.

Thanks in advance for any help