Yes, I tried logging in again. In fact I had been logged out while I was away so what I described was for a fresh login anyway.
So in addition to that I stopped the app and cleared the app data. After starting the app again, and going through the "join existing system" wizard, it now displays the Sources list. However, TV is missing; only line-in is shown. This is the opposite situation compared with what I reported last week, where line-in was missing.
I tried repeating the process and the TV returned to the list. So problem solved, but still no idea what is going on with the app.