UPDATE: Most accounts should now be returning some results, though they may be incomplete. Due to the amount of data being indexed by the search mechanism, new case edits will likely take 1-2 minutes before they show up in case lists.
We are currently experiencing issues with the Case Lists in the Performance Upgrade version of FogBugz On Demand. If you are currently using this, you will see an error message or an empty case list when using case search or filter.
We are in the process of fixing this so that all customers are fully functional again. In the meantime, if you have the toggle enabled, you can switch to Classic UI mode and view your cases that way.