It could be that name used for communication is not right or misses some alias you would usually have for that name. Make sure client alias field contains hostname and FQDN and test network path between them for RPC communication then (for both hostname and FQDN).

The error you see is Windows OS returned error and only parsed by NW when trying to contact client. Perhaps you could also check there is no firewall blocking this communication.

Try winworkr -s {backupserver}.

{backupserver} use the hostname/fqdn with which you configured the backup server.