Windows Server 2008 R2 Adprep fails

I know its way past the point were Server2008 R2 should be running, but my customer only recently decided to update that bad boy. However - when going thru the process of adding a second server (Server 2012 R2) to the pool and trying to proceed the following issue shows up:

Entry DN: CN=ms-DS-Key-Credential,CN=Schema,CN=Configuration,DC=domain,DC=local

Issue begins at line 55: execute denied

Serverside issue: 0x20c3 schema update failed: Attribute in "may-contain" does not exist

extended serverissue:

000020C3: SvcErr: DSID-03260339, problem 5003 (WILL\_NOT\_PERFORM), data 8387

Issue with the programm

Sorry for the bad translation.

Inplace Upgrades also wont work and its a stupid amount of work if i had to start with a new server from scratch, since this exists since the dawn of time.

While performing the update i do have all rights and permissions needed to execute everything.

If more info is needed i am happy to deliver. Just in dire need of help. I was in touch with MS but without success. Googling didnt bring anything useful.