Thanks for the welcome. I'm not really new, I've been reading for months, ever since I got stuck in Divine Divinity for the first time. I'd always been able to find the information I needed to get on, except for this time, so I figured I'd post.
At the end of each dungeon, I ended up in a large room, but way smaller than the other levels in the dungeon, with no other door or entrance than the colourful one I entered through. I checked the walls for doors, shoved packages aside, but no door, hatch, stairs or anything that might lead to another level.
I'm downloading the patch as I type, I hope it'll work. I still have some saved games from when I was in act 1, so I should be able to find out if the patch solves the problem.
