The Update Centre has started showing this error when attempting to refresh the availavble updates:

There were errors checking the update sites: IOException: Could not find JSON in http://updates.jenkins-ci.org/update-center.json?id=default&version=2.462.2

Checking that URL myself, it has Content-Type: application/json but is returning JSONP content, not JSON:

updateCenter.post(
{...}
);