Recently, we need to migrate a domain to a new domain controller. When we tried to demote the domain controller it would fail with a message indicating that another domain controller could not be found.
Eventually we figured out that there were two issues preventing the demotion.
The old server was set as Global Catalog server. We unset that in Active Directory Sites and Servers.
The old server was set to use itself for DNS resolution. We updated the network settings to use the new domain controller for DNS resolution.
Once those two changes were made we were able to demote the old domain controller and remove the Active Directory Domain Controller role.
Hope someone out there finds this useful.