Hi,
I have a similar problem with 2 computers, one with windows vista prof and the other one with windows 8.1 32 bits.
I can login using the server name instead domain name, like this:
domain name_
Zentyal-Domain\User --> fail!!
Server name...
Servername\User --> Login!!!!
I don't know why it happens, but is a problem with users, because they don't know how to login.
I hope someone can help with this issue
I use Zentyal 3.4
Mateu