@cbyrne This is a bug we've identified and are working to solve. In my testing, this happens when a scripting data source has data points AND the data source itself or at least one of the data points is Disabled. When you create a new data source, the default setting is Disabled, which matches the steps you posted on reproducing this issue.
The workaround I've used is to make sure the data source and all data points in it are Enabled and then restarting Mango. One thing to note is that you will likely see an error when you attempt to Enable the data source and click Save. Refresh the data source page after this error and you should be able to confirm the Enabled setting was set properly. After a restart of Mango, you should be able to Validate your script without an issue.
I hope this helps!
-A