I can not go upstairs to continue the quest. Game played just fine until this point.
Have Hrothgar, Shar-Tel, Accalia, Dar & Imoen in my party.

Shar makes her comment about being dupped and then when I click to go up stairs the game saves and while loading it freezes at half the load.

I even reinstalled just in case, replayed the end of the Snake pit and still nothing.

Remove the following file from your OVERRIDE folder:

WTLAKE.TIS

Vlad,I'm not sure if this is interesting for you anymore,since you released a patch for that.But I figured out that this problem is only when you play NEJ 6.91 without BGT.
I tried it 2 times with BGT and had no problem but when I played alone there was the same as GT mentioned above.
Hope this is useful if not ignore me.

Thanks for the confirmation. Yes, I came to the same conclusion, and that is why I asked to remove the water-related overlays, which are somehow broken in the original BG1 and cannot beused in BG2. BGT uses the same overlays, which are okay, so when you install BGT, all these overlays are updtaed with the correct version introduced by BGT. The patch v6.9.2 removes these overlays from the game.