Cleared browser cache, problem still exists in 5.7.7.
I’m scratching my head here a bit and brainstorming on how to isolate what field might be causing the issue.
Any WYSIWYG field will produce the error.
It also breaks the ‘append’ action after a new term is added in edit-tags.php, so any code that is relying on that callback is not working. I had to reload the page as an ugly workaround, so a bugfix would be greatly appreciated.
