Same issue, I don't even know why I got logged out? In the past 3 months all other family members of my account were logged out and I had to cancel their invites and re-send them, that's another issue. I discovered I can't have more than 3.333 songs offline on my phone. And now this??? I swear to God, if one day I'll lose all my saved songs and can't regain them somehow...I'll probably already be a paying customer elsewhere.
This happened to me yesterday. Had to reinstall the app, and then redownloaded several gigs of music over mobile data as I am far away from Wifi. Today it happened again. Not possible to login as I am apparently offline, and all offline songs I downloaded yesterday have been lost, I suppose. Extremely frustrating.
Having same problem every couple of days which sucks because I work offshore and don't always have access to wifi. The whole idea of "going offline" is to be able to listen to your downloaded music, but somehow that seems to make it to where I can't even do that. Then if I restart my phone or the app, I have to relog, but of course I'm offline so I can't. Premium user also, think this might be the last straw for me unfortunately. I've heard there are other pretty decent apps out there for offline music. About time to try one I think.