(I would have commented on JENKINS-20912, but it's closed.)

The issue raised in JENKINS-20912 was closed incorrectly. The root cause is the same as in JENKINS-13465; if you have a parameter with a null value, the JSON serializer chokes and the plugin does not properly handle the error. This appears in version 2.17 and does not appear to have been fixed in any later release.

The steps for replication in JENKINS-20912 still work, the key is adding a string parameter and not inserting any value. However, any null parameter at all will choke the plugin (I ran into trouble with the HipChat plugin not specifying a target room by default).