Okay, I just experimented a bit with debug mode and after a few teleports and turning debug mode off and on again, I suddenly could talk to trader Rekt again. At the Admin Options (in DM) I selected "Reset Quests". I tried to talk to him, the error appeared again. Again I tried a teleport, turning on and off. I could talk to him again and selected "Restock Inventory". And for now I can talk to Rekt, select Tier 5 quests and trade. Looks like everything's back to normal - at least for now. I will watch closely what will happen. If the error reappears I will post the log again.
Very, very strange... This game is full of surprises!
BIG thanks for now!!!