This seems to be an issue, which is quite hard to reproduce. I have made some inquiry to Spotify concerning this, as it seems to be a problem / bug with the library used by QSpot. This issue has indeed a high priority (as the point of offline mode is that you can login without any network connection active), and I really hope to get it fixed soon.

Have you tried deleting / clearing the application cache?

Edit: It seems that the same issue exists with e.g., the MeeSpot and official Spotify clients on the N9. No fix has still been provided by Spotify to fix this problem.