[withdrawn this post since it was not accurate]
I am struggling with the terminology here, not sure if my question makes sense in the context of what UCS is, but can an UCS-based server join a Windows 2K8R2-based AD as backup DC?
Mixing Windows-based AD DCs and UCS-based AD DCs is not supported. You only join UCS servers into a Windows-based AD domain as member servers (and vice versa).
Thanks for your clarification! This was not clear until now.
I suggest that this be added to UCS documentation prominently as it is easy to miss this aspect. Since I now know that UCS can only join as memberserver and not as DC, the documentation on this seems correct, but a causal reader could also conclude otherwise.
May I ask a follow-up question, on why it is not supported, since technically samba4 can be joined into existing AD as DC?
I cannot answer that. Maybe someone from Univention will.