In the meantime... I think I found a fix. First, what I think is going on. That message is about a borked XML that it is trying to load and can't.
But if you launch a direct link into a post in the AC Forum app (say from email, or Hangouts, etc), it will launch the app and load the thread. At that point you can use the app again. One thing I did notice was that the "Portal" page appears to be messed up. Perhaps this page is the root cause of the XML not loading (who knows, I don't dev/test the app at all).
But... any subsequent launch of the AC Forums app will work. The Portal page spins, but you can get to the other sections (Subscribed, etc.)