It happens often depending on the trackers, and their torrent upload policy, some places are really strict and after a couple of hours of seeding/snatches, the torrent gets deleted and everyone who has been downloading it will get the : Torrent unregistered with tracker error.

I confirm that it is related to a torrent that has been deleted, no approved or for whatever reason not anymore on the tracker.

This sucks if you downloaded a 40GB torrent and left out like that without the ability to upload, you may ask a moderator and explain what happened