The notification plugin https://github.com/jenkinsci/notification-plugin is not properly setting the request property "Content-Type: application/json;charset=UTF-8" when submitting JSON via HTTP (see Protocol.java). As a consequence, target systems that rely on the content type are unable to properly process the POST. The patch has been tested and works properly (the target server inspects the Content-Type and loads the JSON handler). Pull request has been submitted with fix.