I tried several things, now. Modded files were now replaced with the old ones, the problem stays. I tried to control Thrace by cheat "control thrace" but that doesnt work. I tried to capture their settlements "capture_settlement xyz" or kill their characters "kill_character xyz". Did not work. Only few cheats seem to work at all. Yesyes, I know the onetime-cheat-rule ;)

Any ideas? I dont want to give up that game, it's about ten hours old.