You should just need to talk to Dotharhar (hallway by specimen room) about the book borrowed from the library to trigger the speech option with Tax to get the key. Try talking with him (again?) and then Tax...
Did you go through all the available speech options with Dotharhar (don't recall if there is more than one), and with Tax? I think you should just be able to ask Tax directly about the book / drugs / imp to get the key.