I am not sure if this will help but enter the MU Shop with the character given the quest. DONT say anything or press any button . . . let the man blather TIL YOU ARE SURE HE is done. If he doesn't recognise quest complete . . . enter the guild with same character AGAIN don't interrupt him let him blather. If he doesn't recognise you now he probably won't. Some thoughts . . . Check your journal under quests to see the status of this quest; did you have this character with you when completing this quest? It is one annoying feature of this game that you must let the folks blather when you enter and it must be the character for whom the quest is for. If this doesn't help take heart, the game goes much faster the second time around (I started over twice) and sometimes you see things you missed that aren't essential to the game but fun.

I have the following problem.
I have alreadey completed the Scabban Quest (Wizard's Guild) with my wizard. Now, my priest is a warlock and joined the Wizard's Guild, too. To reach 2nd guild level, he has to complete this quest as well (!). So I went to Scabban a second time .... talked to him again ... returned to the guild .. but NOTHING! Do I have to kill Scabban ?? Is there a way to reach 2nd guild level with my Priest/Warlock ???

well, one sollution you always have is to reset your adventures. This will at last allow you to solve all guild quests with all chars in your party.
Not the sollution you are looking for I bet...
- Fen

I would try to go back and see if Scabban is in the Crypt. If you go the back way, you should be able to get there in a few minutes. If he is there I say kill him. He serves no purpose as I can see, and if you are low level, he is worth quite a bit of XP.