yakumo1 - I know this is a REALLY late reply, but just writing it anyway, since you may not be the only one experiencing this :P As far as I know there can be 2 reasons.
1.- You tried to change something in the file name. ANY change will mess it up and save it as a zip file instead. So don't try changing the file name.
2.- The place you are trying to save it to extends the maximum file name length. That means the file is in too many folders etc, making the torrent name exceed the maximum signs limit, which will cause the file to say something error. (I can't remember the error message lol), depending on your browser this may change. (The error message that is)