Also when I navigate to : admin/email/templates/system_messages.reviewables_reminder
I get this message :
Also when I navigate to : admin/email/templates/system_messages.reviewables_reminder
I get this message :
I think that happens when you use the new wysiwyg mode of the composer in that input field. I can reproduce it by using the toggle.
Thanks for reporting, we will have a look over the next few weeks. Workaround for now is not to select rich composer in this dialog.