Problem:CU 10 will not install because Forest Functional Level must be raised from 2008 R2.
We had 3 servers: 2 of them Server 2012 and 1 Server 2008 R2.
The 2008 server besides having AD also had Exchange 2010 from which we recently migrated to the new Exchange 2016 server.
I ran DCPROMO on the 2008 Server and removed AD; also removed AD server role. Turned off the 2008 server.
I then checked AD Domains and Trusts and ADSI Edit. Only the 2 current 2012 servers are listed under domain controllers. Raised Forest Functional Level to 2012. Waited an hour, CU10 Update still fails because it says that the FF Level needs to be raised. Waited 24 hours, same error.
Run DCDIAG. See what it says about replication. I'm guessing you've got some errors in there.