I had the same issue last night after restarting my Android phone. I had to do a reinstall of the app in order for it to log back in again which bothered me since I had to be without music until I got home to an internet connection.
Out of curiosity, where is your app saved? Internal storage or SD card? Mine was in the second and I suspect it had something to do with turning my phone off before.
I'm having this same issue and I can't even reinstall the app right now... That's probably a separate issue. But what in the world? I shouldn't have uninstall the app because I got logged out while I was set to offline.
Then after reinstalling I had to reload all music, only to find out it did not load onto my external memory. Then I'm reminded there is no easy way to choose internal over external memory. Bull**bleep** glitches for a 'premium' service.