Can’t Create Windows BDC on my OD Replica?

I have two 10.4.10 xServes, one is the OD Master and a Windows PDC. The OD Master sees the OD Replica which is updating to the OD Master. My second xServe, the OD Replica and wannabe Windows BDC. but when I try CHANGE it's Windows Services Role from Standalone Server to a BackUp Domain Controller (BDC), it fails. I change its Role to BDC I enter the Domain, press SAVE I enter the diradmin username & password (I've tried every other Admin password as well) RESULT = "Error while writing settings (Unable to join the domain)" Then it reverts back to a Standalone Server. ANY IDEAS what the problem is or how to rectify it? Thanks!