Ok, so I've been having this exact problem and I think I may have the solution. It worked immediately for me, so give it a try.
1) Press Windows + R
2) Type in "%appdata%"
3) Look for the BitTorrent Folder
3.1) You may want to create a copy of this. Just copy paste, but rename it BitTorrent.old or something
4) Delete this folder.
5) Download a fresh installer and reinstall Bittorrent from scratch.
After this, it worked perfectly fine for me. I'd suggest you do a backup though, in case it doesn't work for you.
Beware, though, this is a sort of "Scorched Earth" option, since you'll lose all of your current seeds and .torrent files, though the downloaded files should be fine since they're always in a different directory.
I can't say for sure, but I've had this problem twice, and this is usually what works (though the first time I did this by accident). I've recently noticed, however, that this problem crops up when you have WAY too many active torrents, because this usually happens once my seed count breaks 600. I tried reinstalling before, but without deleting the %appdata% folder, and it would just reload all of my old torrents and the same problem would occur.