Ok, after some more troubleshooting, I've sussed out what the issue is and yes, it's a permissions issue (at least for me). We had previously set the "moodle/course:changesummary" to "Prohibit" because of an issue we had with instructors changing things they shouldn't be changing. Turns out that when I toggled this back to "Allow" then the error goes away.
This is new behavior - should I open a bug tracker issue for this?