And I could left the Underdark in the end, not without the same bug. But I found out what caused it: Cernd was in Greater Werewolf form. As I turned him back to normal than everything worked fine. I hope that could help another players in the future too - once again the assertion failure was at the line 899.
Thank you, it worked, I'm in the city If I finished and got out, should I change the variable back to 0? And will the dragon accept the eggs this way, that I hadn't spoken with her before?