When trying to call an APEX class from any flow in my DE org I get an error message:

A fault occurred while executing your script:
Data Not Available: The data you were trying to access could not be found. It may be due to another user deleting the data or a system error. If you know the data is not deleted but cannot access it, please look at our support page.

I'm actually getting the error whenever a flow is trying to invoke APEX, even when I use an example that was posted on a salesforce.com blog. Does nybody have any ideas?