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:

You can often Google the error message and find a Jenkins issue which has been logged by others in the community. Often a fix will be available in a pending plugin upgrade. Otherwise, you can include the error message in your support ticket, leading to a faster resolution of your problem.