Andre, I said what I said because of your statement that "it tried to save the standard object". That's clearly a different scenario then discussed here. If you talk about your own objects only, please go on.
Palle, I reviewed permissions and made sure that were correct (I was even able to save the file a few minutes before), although I don't think I tried to add extra permissions for other groups, only for the current user. I'll give it a try next time. I wasn't able to save the file anywhere (VS and notepad); it was rejected by the file system and not just a particular application. I'm not sure whether your problem was ibn VS too, or you had a problem only with other applications such as Notepad.
The behavior I saw was consistent with having the file exclusively locked by another process, but because restarting the server didn't help and handle didn't show any lock, it must have been caused by something else.