WotLK I am Stucked in a one man Group

[monumenta]

I am stucked in a group and cant leave. I am alone in that group. I can join groups because it says that i am already in a group. If i want to invite it says he/or she is not in your instance.

I tried deleting cache.
/reload
relogging
Game exiting and restarting

Thanks for your Help!

[Blind]

Simply copy paste and enter /script LeaveParty() in your chat and your problem is solved.