Sorry for my slow response, and thanks for your work.
Simply disabling X Theme and subbing out Twenty Fifteen did the trick. I’d updated X Theme a few weeks ago, but the newest version of X did work with the earlier version of your plugin. Which of course doesn’t exactly solve my problem; for now I’ll just shift to the other kind of calendar. Not sure which side needs to be fixed (or both), but there’s a conflict there between the two. I had the same error as mchrisb03 had in the JavaScript console.
If you need me to shift back to the grid calendar on my website so you can explore the error more, drop me a line. Since I moved from a testing environment to a live one, the web address is now at:
https://www.minneapolisriverfrontpartnership.org/?page_id=1070