The problem I'm experiencing isn't new, as a quick Google search has shown me, in fact, there have been multiple people reporting this issue over the last years.
For whatever arcane reason, I was logged out of my account on my Android phone (Samsung Galaxy S3 Mini). I didn't do this myself, mind you. However, since I've always set Spotify to offline mode, I was unable to log in again, as I would always receive the message that I was currently offline and would have to go online to log in - which wasn't possible, since I was not logged in! Now I have exactly two options: 1) Uninstall, reinstall, and download all my 1.000+ songs again, thereby missing my appointment or 2) don't do that and instead not listen to any music on my way.
Dear Spotify team, please, there NEEDS to be a way for users to go online and log in and not have to download their entire library again because they had to reinstall the app. I'm definitely not the only person to ever experience this issue.