Calendar stuck on blue background loading

I’ve used SGC successfully on dozens of sites, and never run into this problem before:

I put the Stout shortcode on the Calendar page, and when I try to load it, the calendar frame (ie, the month and month selectors, the blue background), all load fine, but the calendar events do not load, and a “Loading” message displays, but never loads events.

I’ve tried re-generating the iframe in the calendar, and this is not the problem.

To get the events/calendar on the client’s page, I have put the iframe directly into the text/html editor, and the calendar displays just fine, so it is somehow a plugin problem.

I am running Stout GC on a multisite install, where the other 35 sites have calendars embedded using Stout with no issues. I can’t uninstall and re-install, or I’ll lose all those site’s calendars.

Anyone know why one site might be having these issues? Or have any ideas about what to try?

https://www.ads-software.com/extend/plugins/stout-google-calendar/