Active Directory Forest Discovery was not able to connect to forest %1. Error code returned is: %5. Forest discovery for ...

%11Active Directory Forest Discovery was not able to connect to forest %1. Error code returned is: %5. Forest discovery for this forest will be attempted again on next cycle.

Possible causes include: 
    Network connection problems (server unreachable, DNS problems, dis-joint namespaces),
    server busy or not operational,
    authentication issues preventing site server to connect to the forest.