I recently had the Visual editor not working on several of my sites.
I tried all that was suggested (deactivating all plugins, reinstall wp-admin+wp-includes etc.) nothing worked. Then i tried this :
in a web browser il you have the error : "You don't have the access to the file ..." this is a right management problem.
If you have 644 on the file the problem does not come from the rights on the file but from the rights imposed by your webserver! You may be like me, as i protect lot of things on my websites with .htaccess files, you may have protected it a bit too much !!
Check your .htaccess file and add this for a try :
Allow from all
It works ? Good for you otherwise check with your hosting company, they may have set some rights on the webserver that interfere with wordpress...
Hope that will help...