When you downgrade, Spotify will not allow you to play unless you clear data. This is easy by just going to the settings(inside Spotify) and clear cache. Then, log-in again. If it doesn't work, go to Android Settings > Apps > Spotify > Clear Data. Then Login. If it didn't work, uninstall then reinstall the app. Downgrading Spotify will not remove all the songs saved for offline so you should clear data.