It is related to systemd processing of chroot, as with commenting the User setting, the service start up successfully.
Want to mimic the startup of the service in init script on Debian8 (which is running fine): ~# start-stop-daemon --start --quiet --pidfile "/srv/inst/var/run/rsyslogd.pid" --chuid user:group --chroot "/srv/inst" --exec "/usr/sbin/rsyslogd" -- -i /var/run/rsyslogd.pid -4 -x -- Peter
