Hmm seems I've found the answer elsewhere on this site and it's firmware related:
Their website says: We recently discovered an issue that may cause Autopilot to become unresponsive when first connecting to an aircraft. The root cause of this issue has not been identified, but it appears to be related to the recent firmware updates.
Last Update: Jun 10, 2017 3:22:58 PM UTC
Annoying. Looks like I won't be able to use the app until they fix this issue.