I've got this error when I try to edit an article with Visual Editor. This error does not appear when I write a new article.
So, LapisLazuli33 wrote good pieces of information about rewrite rule. And so I found out, that my .htaccess auth file makes the problem:
AuthUserFile /www/htdocs/*/*/.htpasswd
AuthGroupFile /dev/null
AuthName 'please insert password'
AuthType Basic
require valid-user
When I rename the file, I can use VisualEditor. But this authorization is important for my NGO. Is there a way, I can use both?