Spotify doesn't start anymore. WP 8.1

[pinnarandolf]

The problem now happens the second time.
The first time I was able to solve it by rebooting my phone.
Now the problem seems a bit more serious.
The problem occurred, when I was playing via AUX/Earphones on another device. I noticed that I was not able to adjust the volume anymore. I tried on the phone and on the other device. Even when the phones volume was at 0/30 it was still playing on the same volume than before. I unplugged the AUX cable and tried again. Still the same volume. I tried on the phone speakers. Still the same.
I rebooted the phone. Spotify doesn't start anymore, no icon on the tile.

Phone: Nokia Lumia 635, Windows Phone 8.1

[pinnarandolf]

Probably useful too:
I rebooted multiple times
I also removed the phones battery
I was in offline mode
I have an offline library of about 5GB