Bookwyrm627: I suspect you're in trouble; assuming you've been clearing each level as you go, you'll have to enter level 5 again at least once more (and probably several times more). Do you get the error regardless of whether you enter level 5 from level 4 or level 6?
Enebias: Yes, I get the error both from level 4 and level 6.
Let's suppose I restart the game: would I be safe if I kept just the bare minimum number of items both in my inventory and on the ground? Is there the chance that the bug will appear even under those conditions? Would "cleaning" the floor by throwing things in the water (or lava) help?
I'm really liking the game and I certainly would prefer to finish it, yet I just don't have the time to keep on restarting until I have luck and miss the error!
Since I don't think that support could fix a notorious bug, I guess this may be the only solution.
I think you may as well restart, then. You'll need to traverse through level 5 at least a few times more, in one method or another, in order to finish the game.
Honestly, I don't recall ever encountering what you describe, and I've played on and off (gog and non-gog versions) over a number of years. When I was young, I would stockpile pretty much everything I found on a level into an area I'd label a "storeroom" on that level, and occasionally move stuff to lower level store rooms as I advanced. More recently, I try to make a point of only carrying things that will be useful (helps that I know everything I'll need, and a number of things I'll want, already). So you shouldn't have to worry about throwing everything you don't want into the water. Did you spawn a lot of things (read, "food") with spells or anything? If not, I'd think a restart should cover you (perhaps a re-download and re-install just to be safe).