How to put a Windows 2012 server into a Univention domain

I hadn't heard of Univention before. It looks like it's a distribution of Linux and Samba with some "baked-in" management tools.

It looks like you can install either Samba 3 or Samba 4 (per this screenshot. There appear to be known issues with Windows Server 2012 joining a Samba 3 domain, so that may be your problem.

I'd compare the DNS settings on the Windows Server 2012 machine to a known-working machine. DNS problems are typically the cause of issues joining Active Directory domains. If you haven't, work through the domain join troubleshooting document from Univention.

Edit:

In a Samba 3 environment, which emulates a Windows NT domain environment, a couple of defaults in Windows Server 2008 (and newer) need to be changed. The following registry merge changes the default behavior to allow for joining an NT-style domain:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\LanManWorkstation\Parameters]
"DomainCompatibilityMode"=dword:00000001
"DNSNameResolutionRequired"=dword:00000000