Let me reiterate by saying ePO has always been installed on my Windows Server 2012 server. There was an old 2003 server that was the primary domain controller, and my 2012 server replaced it today. The ePO services had no issues before the 2012 server was promoted to the primary domain controller.
Well I managed to resolve this issue. I had to edit the properties of the affected service and change the "Log in as" account to the Domain Administrator.
yes - the reason is mainly that your DC now uses the group policy (default) domain controller policy (and I'm quite sure that there is a setting that, by deault, only specific accounts are allowed to run services).
So - I woul not recommend to run epO as domain admin - either edit your policy, so that your service account is allowed to run the service - or (and I'm not sure if this is default) - running ePO as local system should work, too (?)
btw. I would not reccmmend to run ePO on a DC (if you only have one server for all - well you have to)