Unable to add child domains, it says "Unable to get domain DNS/FLAT names, configure domain with valid credentials".

ping test from AD Manager plus server to child domain's DC works.

port 53, 389, 3268, 445 are fine.

DNS resolutions seems ok

Bidirectional trusts also seem normal.

It used to work even when no authentication is given. Now if I give wrong credentials it will instant fail, correctly stating that the password is wrong. If I input legit enterprise admin credentials, it will give the error above after around a min. 

How should I investigate further and how could I turn on the relevant log? Thanks.

Best regards,

Philip