When PING was blocked on your network, BackupPC can not backup hosts because it will stop the backup process when there is no ping response. Here is a workaround to make the backup happen:
Change the PingCMD (ping command) in the host setting of BackupPC webinterface:
1. Seclect the host then click "Edit Config" link.
2. In Backup Settings tab, PingCMD section, check "Override" and replace
"$pingPath -c 1 $host"
3. Click "Save"