I didn’t recognize that you use Nethserver 6 that’s why netstat looks different.
There seem to be problems with centos 6 support of urbackup (very old urbackup version is used):
[root@server ~]# urbackup_srv --version
CServer version 0.1.0.0
[root@server ~]# urbackupsrv --version
UrBackup Server v220.127.116.11
Copyright (C) 2011-2018 Martin Raiber
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Sorry, I don’t have an easy solution now, I couldn’t find an rpm for centos 6 so you may use/update to NS7.
I reproduced it on a NS6 VM with exact same error. It could not find the backup server, I tried several restore CD versions.
Only way I found is using other repos and compiling a newer version of urbackup.