Event 13508 in the FRS log is a warning that the FRS service has been unable to complete the RPC connection to a specific replication partner. It indicates that FRS is having trouble enabling replication with that partner and will keep trying to establish the connection.
A single event ID 13508 does not mean anything is broken or not working; simply look for event ID 13509 to make sure that the problem was resolved. Based on the time between event IDs 13508 and 13509, you can determine if there is a real problem that needs to be addressed.
Because FRS servers gather their replication topology information from their closest Active Directory domain controller (itself on a domain controller that is also an FRS member), there is also an expected case where a replica partner in another site will not be aware of the replica set until the topology information has been replicated to domain controllers in that site. When the topology information finally reaches that distant domain controller, the FRS partner in that site will be able to participate in the replica set and lead to FRS event ID 13509. Note that intra-site Active Directory replication partners replicate every 5 minutes. Intersite replication only replicates when the schedule is open (shortest delay is 15 minutes). In addition, FRS polls the topology in the active directory at defined intervals — 5 minutes on domain controllers, and 1 hour on other member servers of a replica set. These delays and schedules (and especially in topologies with multiple hops) can delay propagation of the FRS replication topology