IIRC this happened to someone else as well, but I don't remember any details and could not find the topic in a quick search (it may have been in the old forum).
You can bypass the stairs using the procedure in the following topic. I'll update it with the coordinates for the start of the first level. Arhu should point you to a chest with your belongings in a cutscene after you take the stairs, so hopefully hex teleporting to that place will also trigger the cutscene (if it doesn't get triggered, it might not be a problem).
Teleporting Anywhere, via hex editorThere may be a problem with the world.x0 file in your saved game folder (the world.* files keep track of walls, trees, buildings, etc and items left directly on the ground). If so, hopefully it is isolated to just that set of stairs, and bypassing them will allow you to proceed normally (you will not need to return there).
If there are more serious issues with Iona's dungeon (a couple people had missing walls, for example), you could make a copy of your latest save (or save again under a new name), and in that new save folder delete the world.x0 file. The game will replace that file with the default version when you load the save, which should fix Iona's dungeon, though it will also remove any items you left directly on the ground, and re-seal any quest opened locations. See the following topics for more info, if you need to.
Game crash upon going to Verdistis /
Game crashes when I try to return to VerdistisCheck your messages (click 'My Stuff' in the top menu bar, then 'My Messages').