Did you try clicking on the locked door after Tax says that he has given you the key? (I assume so, but it can't hurt to check) The key should have been placed in your hero's inventory, but could be hidden or obscured by something else.
If you had your DK talk to Tax, perhaps your hero would have better luck?
If you do not have the key, it is possible to get a replacement copy (I would have to load one of my old saves at that point to get the required
inventory files).
Alternately, you can bypass the locked door either by
hex teleporting or using the technique described in the
armory trick in act 1 topic to get a summoning doll through the wall, and have it enter the BF, then exit to get your main characters through (repeat the process to get out of that room again, when you are done in the maintenance tunnels).
Let me know if you would prefer the inventory files method.
If you need more help we should probably take this to a new topic, as it is no longer about the patch.
