Uncaught TypeError: tinyMCE.addI18n(...) is not a function

In the browser console, I am seeing the message below when editing any chapter:

Uncaught TypeError: tinyMCE.addI18n(…) is not a function

I am using the latest versions of the plugin and McLuhan Theme.

I also see this message on Kardiovaskuläres Manual 2023 – Simple Book Publishing

Thanks for reporting @t.dumm. I’ve filed as a new bug here: Uncaught type error for tinyMCE.addI18n function · Issue #3478 · pressbooks/pressbooks · GitHub