My wife and I both have Windows Phones, and both share a Spotify Premium family account. We have tried this in FOUR different handsets (HTC Arrive, two Lumias 925, Lumia 650), and we get the same bizarre behaviour.
When we log into Spotify with my account, the app works well; it is fast, stable, and has all the features working.
When we log into Spotfy with my wife´s account, the app is completely unusable, slow, sluggish, constantly crashing and loading, not playing any sound.
We have loggged in an out, reinstall, factory re-installed the stupid app, nothing works. My account flies, hers just doesn´t work at all. Again, Premium family account. This ONLY happens on the Windows Phone app; Spotiy works fine for both on Android (tested it on a phone and a tablet) and desktop.