I saw this exact error today and the fix was to use a different AD credential to join the domain. The administrator account the customer was attempting to use did not work for some reason (we did not hunt for a root cause - but could be a rename of the default Administrator account).
Thanks Grubbs91 for posting your fix.
Original Message:
Sent: Sep 25, 2019 05:45 AM
From: Jack Law
Subject: Problem to Join to Domain
Hello,
We had this exact same issue after migrating ClearPass from a Hardware appliance to a virtual appliance using backup/restore. After migration to the virtual appliance tried to domain join the nodes to the domain but getting the same error null.
The credentials that were being used to try and join the domain didn't have enough privileges. Used a set of credentials where the username was a member of domain admin group. Node joined straight away. Hope this helps.
J