Are you talking about claiming the extra reward from the Attoraxian chest at the end of the questline? If so, I had the same problem. I re-did the final boss fight which allowed me access to the chest room again, and I was able to complete that bonus quest. Hope that helps!

I dont see a possibility to geht straight to your reward, without clearing the whole dungeon again. Feel sorry.

Long story short I exit the dungeon w/o picking up the chest. I have the key but don't know how to get there. Is it really not possible to get back in? :')

Possible to enter the dungeon reward room?