Hi Yigit,
there is only one plugin (NextGEN Gallery by Photocrati). I deactivated it, but have still the same problem.
I’m using 2.9.2 and WP4.0. The advanced layout editor is not working anymore.
The text-editor shows HTML-sourcecode, but the visual-editor shows an blank input-field. And this is strange: the buttons for formatting (bold, link, etc.) appear in the text-editor-mode, not in the visual-editor !
It’s impossible to work with, please help.