When submitting a configuration form to Jenkins, I get a 500 error saying This page expects a form submission but had only

When submitting a configuration form to Jenkins, I get a 500 error saying "This page expects a form submission but had only {}"

Any version of Jenkins

Generally if you get this error, it means that you need to look at the Javascript console (View -> Developer -> Javascript console) for more information on how to diagnose the problem. Often, you will see a 500 error encountered while executing some Javascript like this one:

You can then click on the URL in the error message to get the root cause.

Include the symptoms and error message in a CloudBees Support ticket so that we can help you diagnose the problem.