I think a reload from an older game may fix the flickering textures? I had this issue too on one playthrough - I can't remeber if I was able to fix it. On my first playthrough , I picked the lock anyway - just walk carefully around the map triggering perception checks and you should find it. I sometimes can't be bothered with puzzles, so I am glad they provided that as an alternative.
The lock is one of the rings near the side that exits to the Underdark. Can't remember precisely where, but just walk about and its hould show that you've spotted something. I think the open check is quite hard though....spells like guidance can help with that.
