"The operation failed because: the attempt at remote directory server serverx.customerdomain.local to remove directory server CN=ServerX,CN=Servers,CN
was unsuccessful. "Access is denied"
The account which i was running dcpromo with was both Domain Admin and Enterprise Admin, i could ping the other DC's, i checked with ADSI edit that my account had the rights to delete the DC. Everything seemed ok.
So what was the problem? I had a look in Sites and Services and found the solution to my problem. The DC had the checkbox "Protect object from accidental deletion" ticked. I unchecked the tickbox and could then run the dcpromo without any problems.