[Samba] adding a win2008r2 server as an additional dc to a samba4 dc created domain

[Neil Price]

I'm using 4.2.2, sernet, on Debian Jessie. Bare bones recently created dc. I can join workstations, 
add users, groups using the samba-tool and and the microsoft remote tools.

Now I'm trying to add a freshly installed 2008R2 as an additional dc.

All seems to go fine (I get an error about dns delegation which I think I can ignore), it finds the 
domain and the site name,  but the join fails.

"The attempt to join this computer to the "domain.tld" domain failed. This operation is only allowed 
for the primary Domain Controller of the domain"

Logs show nothing significant. Nor does Google. Any ideas?