Sounds like your dc demotion didnt go through properly - you need to do a Metadata cleanup to remove the old server properly from Active Directory. If you dont remove it cleanly you will have gpo niggles until you do. How to remove data in Active Directory after an unsuccessful domain controller demotion - is the MS article on this - make sure you have good backups and a local systemstate backups of all your DC's before doing this stuff.
It's also worth checking secure channel for all your domain controller servers as well - do this from each DC server - log on as a domain adminstrator and from the run command try to access the c$ share on every other domcina controller e.g.
Log onto server 1
start - run - \\server2\c$
start - run - \\server3\c$
start - run - \\server4\c$
Log onto server 2
start - run - \\server1\c$
start - run - \\server3\c$
start - run - \\server4\c$
etc
If you get prompted for username and password at any point the secure channel is broken between the servers and needs to be sorted BEFORE you try to fix the failed dcpromo.