I think this bug has already been reported, but it is the most annoying I have encountered in the game. It already happened to me 4 or 5 time in the last year, and it is always of course a definite game over...
It happened to me in different situations, from different dungeons, at different char level, so I see no evident correlation that could help to fix it...
In a similar way, a couple of betas ago, I also had the entrance of a dungeon that did not appear (or that disappeared, I don't remember). It was charred scar, and there was no entrance in the usual place (or any where else), despite the fact that I had the important news quest. The rest of the map was OK, but it was again a game over.
