After getting stuck in this quest, i solved it by loading an older safegame and repeating the fight with Raedric. Btw. the first repeat was successfull...
I entered the throne room through the chapel, went to the dais and killed Raedric. After the fight i picked up all the loot (in no hurry) and left the dais in a direct line toward the double doors.
I dont know if this will help someone, but repeating the fight seems to solve the problem sometimes.