Can’t edit files in Appearance Editor. 2.9 problem?

My blog is https://www.mandyjill.com.

I recently upgraded to 2.9. Now, when I go to Appearance Editor in Admin and try to edit any pages, it gives me this error:

Oops, no such file exists! Double check the name and try again, merci.

I can always download the files and edit that way but it’s a pain in the butt and I want to be able to edit pages/css via WordPress admin panel.