The Directory Server detected that the database has been replaced.
The full error is as follows:
The Directory Server detected that the database has been replaced. This is an unsafe and unsupported operation. The service will stop until the problem is corrected.
User Action:
Restore the previous copy of the database that was in use on this machine.
In the future, the user is strongly encouraged to use the backup and restore facility to rollback the database.
This error can be suppressed and the database repaired by removing the following registry key.
Additional Data
Registry key:
System\CurrentControlSet\Services\NTDS\Parameters
Registry value:
DSA Database Epoch
This ended up being a DNS issue between the two AD DC's. First AD DC had a DNS server as well so had to have itself as a DNS server. So enter first DNS server's IP into the DNS 1 field and enter the router's (usually 192.168.0.1) into DNS 2 field.
Likewise for DNS 2. Enter the IP of the second DNS server into the NIC DNS 1 field of this second DNS / AD DC server. DNS 2 should be the main router 192.168.0.1
Cheers,
TK