A fix for a significant (fortunately rare) problem for the iOS app is on the way. Previous build crashed on start, but only when a significant amount of listening (say at least weeks) was made on the "standard" app. I think I fixed the problem, but I cannot test it (because I should wait some weeks) 🙂 
So, if someone is available to help, s/he can get the test build fromhttps://testflight.apple.com/join/ieYKbfZv (follow the instructions there).
If it crashes at start (or if you want to go out from the test build path), you can always get back to the “normal” build by deleting the app and reloading the app from the App Store app.
Opting out directly in the TestFlight application or reloading the store version returns to the "non-test"/official build. No information are transmitted by using the beta but the crash logs (if any and through Apple) and whatever you decide to share as feedback.
If you test, let me know of any failure (and/or successes) here or directly through the app (preferences -> feedback). Thanks!
For the records, the test build also fixes a problem in the Apple Watch app, that was sometimes, stopping the music at the voice breaks.