Doubly sad because I had to play through the game TWICE (once with the quest download, and once with the different version that comes with ZC v2.1) to confirm that this is indeed a problem with the new game version. Since the quest is not editable and I don't have editing experience anyway, I can't really say anything about the nature of the problem.

It seems like from what I remember, every few seconds the game is supposed to shift you from one map to another, each representing a different phase of the extra dungeon emerging from the ground. In v2.50, instead, it seems to shift you to the same map over and over again every few seconds (noted by your character being teleported to a standard position every so often, but all other displayed things remain). As a result, you're stuck in the first frame of the emerging dungeon "animation".

I was looking forward to reviving interest in this epic challenge by posting a hint guide for it, but now I can't get to the last part...oh, woe and unrequited love!

...In other obviously unrelated news, I beat Demo Quest EX in one sitting and without saving today: 5 deaths, 5:08:12. Beat that!