BackupPC will not start a backup if it does not receive a ping reply from a client.
Here is how to backup a host when ping is blocked, eg if the client to be backed up is behind a router.
In the BackupPC webinterface, click the client host name, Edit Config, go to the Backup Settings and change the setting PingCmd ($pingPath -c 1 $host by default) to
/bin/echo
Don’t forget to check override and click the Save button to apply the change.