I don't think I've ever had an extension give me as much trouble as this one. It didn't work and then it worked (and I documented how I got it to work; apparently not all the necessary templates were imported) and then it didn't work anymore (I guess after I imported more templates or something). Good ol' "Script error" with its specifics that aren't described in any of these postings. Anyway, I'm going to keep plugging away at this troubleshooting, and then post the details of what I encountered and how I solved it (because for one thing, I might need the info later too). I did try some of the solutions specified for similar problems; those didn't work, and I'm upgrading all of my extensions just to be safe. I got Scribunto to work on a brand-new MW installation so I know my extension code was good.
I hope others will do the same (i.e. document problems and solutions); even if you figure out how to solve your own problem without needing to ask a question here, please post in the article a description of the symptoms and what you did to fix them. Hopefully by working together we can put together documentation that will help others (and ourselves, if we forget later) figure out how to make this diabolically frustrating extension, with its mysterious error messages and usually unhelpful/uninformative error logs, work properly. Thanks.