I'm having the same issue with all three of my hubs. It looks like it's not able to http post to the hubs.
groovyx.net.http.HttpResponseException: on line 214 (initialize)
As a quick test, I fired up Postman and tried the same call and I'm getting back a HTTP 417 code. All three respond to pings and I'm able to access them through the Harmony app. Plus, all three are setup with the XMPP "feature" setup. I can also access them through the Harmony API (using GitHub - maddox/harmony-api: 🗼 A simple server allowing you to query/control multiple local Harmony Home Hubs over HTTP or MQTT).
All three of my hubs are on firmware: 4.15.250, app version 5.6, build 14 (if that helps).