If you install the Parameterized Trigger plugin in an older version of Jenkins, you can get a 404 in the validation area for “Build after other projects are built”. This is because Jenkins.instance.getDescriptor('BuildTrigger') (used from the /job/downstream/descriptorByName/BuildTrigger/check?value=upstream URL) may return either a hudson.tasks.BuildTrigger.DescriptorImpl or a hudson.plugins.parameterizedtrigger.BuildTrigger.DescriptorImpl, apparently at random.
4d33b6a should have fixed this in core 1.480. However it would be better for this method to actively check for ambiguities so similar problems do not recur.
Code changed in jenkins
User: Jesse Glick
Path:
changelog.html
http://jenkins-ci.org/commit/jenkins/4b64f13d3c2c754f988ebe472cd4e8d7839069eb
Log:
JENKINS-14995Noting, retroactively.