I do realize multiple posts re:this issue. On the ones that offered ideas to correct none worked on the one site I have broken.
I run 3 sites,and an extra for test
each has own instance of PHP Mysql WP.
started with 3.9.1
upgraded to 3.9.2
only one site has this signature the rest work with same plugins / themes.
SIGNATURE (A): ie,what is broke
Site seemed to be fine for 8 days.
Noticed I could not access text mode in the visual editor, or add a link, or launch the media button.
All in the page add.
I followed all the posts and performed plugin theme browsers.
I did delete the instance, rebuilt from scratch. and the signature A went away. Of course, when I restored from backup the signature returned. the media files would need to be deleted and re added. Seemed links were hosed.
other signature (B): The theme customization would not allow the use of. displayed the menu not sub menus. (this seemed to go away when I removed 2 very large jpg files from the media library.
that is all I can add. If you need logs etc let me know. Again 3 sites out of 4 same setup only one has issues.