one change to the servers - one of 3 domain controllers had be demoted to a member server 2 weeks ago and before the problem. (existing and current other 2 DC's not changed)

I know what you're probably thinking and I did initially think that this could be the cause of the problem, however, the demotion was completed without issues at the time and the replication and AD seems to be OK. There are also no errors/problems in the event logs for all domain servers.

Do you think I've missed something here?