I just went there to kill 50 of them and had to learn, that there are no tholians at all. It looks like they stopped spawning. Now i cant complete the mission.
I also had a problem with a medium mission yesterday where i had to close some rifts. I stopped at 7, went to sleep. This morning, i wanted to continue and still had the counter at 7 but could not close one rift. (Maybe because they were all closed by other players but did not open again and that would explain my first problem too?)