Don't bother about missing groups. This is for other linux os where it is needed, so failing in pi is ok here.
run
ps aux | grep tier
to see if it is running.
Check /var/lib/Repetier-Server/logs/server.log for some hint's on what went wrong.
Localhost on pi will not work on the old pi, that will crash the server and pi is not fast enough to run webpage. Maybe Pi 2 can do it. So try from external device only!
/etc/init.d/RepetierServer restart
to start again.
