Actually, I found the culprit... it's the "Enable Scripting" checkbox. I went through one of the other folk's .caps deleting things one by one and returning it to as close to a blank .cap as I could, and when I unchecked that box it ran fine.
I confirmed it with a new, blank .cap. I did nothing but tick that box and I got the error when I ran it.
Is this a bug, or do I just need Python installed or something?