Screen shot of the error message
I was expanding some subcategories in the category tree, on Simple English wikipedia, when I got what you can see in the attached image. I think it would be a good idea to update the CategoryTree extensions (or maybe the AJAX functions of MW itself, in a way that a more user friendly message is shown in such (although rare) occasions when the server doesn't returned the data in the expected format.
Version: unspecified
Severity: minor
Attached: