Is it entering the playhouse that causes a problem or just getting near it?
If you were in the playhouse previously with that character, went through the cellar and came out the hatch on the opposite side of the street (near the soldier burning undead corpses and the healer wanting to study them), you could try getting back in that way.
If you start a new FoV character can you head straight to Lanilor Lane and enter the playhouse?
What about using the Developer Mode version of the game, opening the cheat window (F11) and teleporting there, using 'goto Region' Exp_AlerothCity Exp_AC_FightHouse03 (which is the fourth in a list of such locations, with the numbers cut off).
Did you try re-installing the game, or verifying local files for the Steam version? (especially if you have a problem with a new character)
