It is now starting to roll towards the end of the 1 month trial RIM is offering with BBM Music and the Beta Zone app is starting to expire users accounts. The problem is that during the Beta Zone period it is not possible to actually upgrade to a premium account. I ran into this "expired account" a few days ago but after fiddling around it just worked again. I ended up just escaping all the menus and the app worked but AgentBlackBerry over @BBOS was kind enough to write up a simple FAQ on what to do:
If you get this error message simply dismiss all the "upgrade to premium account" windows, fully close the app by selecting "Close" in the menu and not just pressing the back or END button. Upon reopening the app you should see it refreshed, all your songs available again and a new expiration date.
Also worth noting is that every month RIM refreshes your count of changeable songs with 25 changes. You should make these changes before you do this because it will reset the count to 25. Let us know if this works for you!