First, thank you for the work on this software and hardware!
I think that I am experiencing this same issue. I updated the iPhone app to version 1.7.9 recently – NO other system changes – and now the app says “Unable to connect to Home.” I can access the web UI without problem.
Again, the only thing that has changed is the app upgrade.
Edit: I am using a Pi version installed on top of Raspbian. Not able to get a shell to check version, but it was pulled from master sometime in June. Can get version info later.