While looking on the other posts, I found this one
Changing source(s_sys); to source(s_src); does solve the problem. Warning about config file too old is still there, but service is able to start.
sudo nano /etc/syslog-ng/conf.d/librenms.conf
source s_net {
tcp(port(514) flags(syslog-protocol));
udp(port(514) flags(syslog-protocol));
};
destination d_librenms {
program(“/opt/librenms/syslog.php” template (“$HOST||$FACILITY||$PRIORITY||$LEVEL||$TAG||$R_YEAR-$R_MONTH-$R_DAY $R_HOUR:$R_MIN:$R_SEC||$MSG||$PROGRAM\n”) template-escape(yes));
};
log {
source(s_net);
source(s_src); <===========
destination(d_librenms);
};