Well, the error we are getting
"External DB user invalid or bad password" means External DB (i.e., Windows or any other external db configured with ACS) sent the message back to ACS server the user who is trying to authenticate, either username or password or both provided by user is not found/matched with the users present in the database. ACS talks with windows AD in terms of codes, and ACS interprets these codes and displays them in the passed/failed logs of ACS server. Most of the times this could be find as permission issue with ACS installation.
As you said that other devices are working fine with same account and same ACS box. I would also like you to retype/double check the shared secret key on both the sides (ACS and AP).
Hope this helps.
~Jatin