I already had a working Jenkins and Gerrit setup on AWS, as a shortcut to set up a second Jenkins server (call it J2) I created an image (AMI) of the existing Jenkins (J1) and proceeded to change a few settings of the new one. At one point while configuring the J2 I ended up on the J1 URL and accidentally made breaking changes to J1 without even noticing I had moved accross.

A little investigation revealed the Gerrit Trigger settings cog to be the culprite. When you go to Manage Jenkins > Gerrit Trigger > "settings cogwheel", it goes to [Jenkins URL from global settings] + /gerrit-trigger/server/SERVERNAME/ – If the Jenkins URL is wrongly setup this can lead to a broken settings page or worse (see why above).

This is easy to avoid once you know the reason, but I find it strange that this occurs here when all other navigation in the Jenkins management pages are relative URLs which works fine even if the Jenkins URL is wrongly configured.