Alright, so as it seems Jenkins has moved to default POST in the callback setting checkmethod as instead of GET. What does that mean is when you load the configure UI page each plugin has a callback to its own page to obtain additional information for the UI such as validation or contents for dropdowns. Since some plugins are outdated and or don't support POST methods at all the response is failing with 404 which breaks the UI. That doesn't mean that the overall functionality of the plugin is broken, but it seems that it was a bad assumption that led to backward compatibility. The move seems to be related to efforts in relation to CSRF protection.]
I don't expect an easy fix from Jenkins here and you have a couple of options:
1. Fork and update each plugin that suffers from this issue. You could use developer tools for example while the page is loading to find which /checkSomething pages are broken with 404.
2. Put a proxy with redirect certain queries to the backend from POST to GET
3. Wait for a general fix, which I don't expect anytime soon.
I hope that helps!
Hey pbenz have you managed to find a solution? We had identified that potentially this is related to how the Jenkins engine handles 404, specifically in our case it was related to the use of outdated and broken plugins such as pyenv and rubyenv.