10.4.11 client don’t authenticate on replica server 10.5

We' ve got a 10.5 OD Master and a 10.5 OD Replica. 10.5 Clients (mobile) can log on on master, on the replica (if the master is down) and also locally (if all servers are down) 10.4 Clients (mobile) can log on on master and locally (if all servers are down). If the OD Master is down and OD Replica is up our 10.4 Clients can't log on. Any Ideas ? I think Replica Server is OK, Leopard Clients use it with any problems. Thanks, Sandra