PROBLEM: Anytime Spotify decides to logout and the app was in 'off-line mode' you will no longer be able to log back in. It will just continually give you the generic check connection message, or the login button will just be stuck in the active phase where it turns white and has the text "Logging In" .
FIX: The only fix is to uninstall the App, re-install the app, and then go back through and re-select everything you download for offline listening. Then wait for it to re-download.
FREQUENCY: Undetermined. This is the 5th or 6th time this year its happened. It happened twice in one week a few months ago.
FURTHER COMMENTS: This is a rather serious BUG and it's apparently been ongoing for several years. Seriously if my other half didn't like spotify so much (they don't need offline mode and therefore don't have this problem) I would have jumped ship to Anything else. The first 1st Gen Zune system was less glitchy and more reliable than the Spotify App has gotten over the last few years.