I have Spotify on my Nokia N9 MeeGo phone - and I have used it without problems for a long time. Then, suddenly, I am asked to log in every time I am offline - and only then. As I am offline, I cannot log in, and am stuck without the music I have synced to it. If I try to log on when I am online, I am usually not asked to log in.
If I make sure to have Spotify open when I am online, Spotify usually still works when I go offline, as long as I don't close the app.
The app itself suggested to reset local data, recently (after the problem began), but it didn't help.