I had a similar issue when I tried to fix my signature (the gap in between the bottom line of text and the spoiler bothers me), and everytime I tried to save it, it gave me an invalid file error. I think it resolves itself, because it may have something to do with the host site not functioning properly.
Are you uploading a gif? Are you uploading a picture or something that is bigger than the standerd signature size? That may be one of the issues.
Yes, this is fixed now. I didn't notice there was a separate thread for it.
It only didn't work on the default TBT theme.