Getting tinymce errors in Popup Pro page

When I try to edit a Popup in Popup Pro, the activity circle spins for a long time, then I get the following errors:

Failed to load resource: net::ERR_CONNECTION_RESET
Uncaught ReferenceError: tinymce is not defined
Uncaught ReferenceError: tinymce is not defined
Uncaught ReferenceError: tinyMCE is not defined
Uncaught ReferenceError: tinymce is not defined
GET https://widgets.wp.com/languages/notifications/en_US.json 404 (Not Found)

I am using the Chrome browser on a Mac. I'm as up-to-date as I can be on all software on my Mac and in my WP installation as of today.

I am running a WP multi-network multi-site, and there's no issue in my other sites. If there's some easy way to do a comparison to see what's different in the setup of this one, let me know and I'll do it.