We migrated a while ago from SBS 2008 to Windows Server 2012 R2. The new Server is also the Certificationauthority. But when I want to renew a certificate, a get the error messsage "Error: RPC Server is not available".
Since the URL shown in the error message points to "oldserver.domain.local" and not to "newserver.domain.local", this makes sense.
How can I change the address to the new server?
The problem is that you didn't decommission old server and its reference is still exists in Active Directory. You need to remove stale records. Here is a CA decommission guide: How to Decommission a Windows Enterprise Certification Authority and How to Remove All Related Objects
I suspect that you already removed CA server itself, so you need to perform sections 6 and 7 in referenced article.
