Hi--my friends and I ran into this same issue.
We saved at the end of the night, immediately before entering the final chamber.
The next time we resumed play, several (all?) of us left that chamber to stock up on potions, scrolls, respec into loremaster to identify items, etc, since it was implied that we wouldn't be able to do so after entering that chamber.
After we did this and fast-traveled back to the...crypt(? Whatever waypoint was closest) and walked towards that final chamber again, we also could not enter the final chamber. We would try to enter, all accept the ready check, and....nothing would happen.
In order to fix this, we had to reload our save file from before we fast traveled out of the penultimate deathtrap chamber and, without fast traveling away, initiate the ready check to proceed into the final chamber.
This allowed us to enter the final chamber and complete the main storyline.
The common factor between our experience and OP's is that we could not enter the final room if we fast traveled out of the deathtrap chamber. I think this is what caused the bug, and if anyone else runs into this, loading a save from before you fast traveled away from that chamber should fix this.
This is, of course, a little frustrating, and something that I hope will eventually be fixed for future players!