net rpc join -U administrator
Joined domain DOMAIN.
When you did that before, you were joined to the domain if you see the response it gave you.
So after that, when you did wbinfo -u, it should have listed the users on the server2008 domain, which it looked like it did.
So therefore your server had joined the domain at that point.
I suggest going back over this. When you use the above command, be sure to use the "%" symbol straight after the word administrator, then use your server2008 Administrator password, so the command should read:
net rpc join -U administrator%12345678
where "12345678" is your actual password.
Don't forget to
signal-event workgroup-update
when it has joined the domain.
I assume that you are doing this in a test install and the name of your server2008 Domain is DOMAIN.local - you can forget the ".local" in your workgroup setting in server-manager.