Novell GroupWise 2014 R2 SP1
You have upgraded / installed all components on the server. GW monitor agent runs fine and you can hit http://<ip>:8200 page. When trying to hit http://<ip>/gwmon .. you get a language selection page but after clicking on GO, error 5xx appears.
When you run a configuration part of the GW monitor application part and you choose not to include / specify any certificates, this is the cause of the problem.
Edit /var/opt/novell/groupwise/monitor -> gwmonitor.cfg file.
Search for a section "Provider.GWMP.Agent.isSSL" which is by default set to true. Change it into false, save the file and restart tomcat.