When you run setup to upgrade Exchange server 2013, it checks prerequisites and one of its actions is to contact Active Directory to check schema version for possible need to update. In my case the problem was, that error 0x51 occured. I was checking what is happening. The first thing was to run netdom query fsmo command to gather which FSMO roles were placed on failing DC1. As it was during Schema version check, I was mostly iterested if Schema master role is present there…. and it was, No I was wondering what is the problem. I went to OU=Domain controllers in ADUC and checked DCs. I realised that Schema master role is running on non GC domain controller.
Solution: To run setup successfully seize FSMO Schema master role on DC with GC in the same AD site as Exchange server.