This problem usually occurs in a mixed subnet environment on a local LAN, VPN or similar network. This a case where autodiscovery of the server does not work and may conflict with attempts to manually add it.
To resolve the issue, open a desktop session on the problem server.
Using a plain text editor, in the /FileReplicationPro/etc/S2S.properties file. Find the line:
2s.enable_auto_discovery=true
Change it to false and save it.
Restart FRPRep service on the problem server and after a few minutes try to add it to the management console.