See this thread:
http://forum.joomla.org/viewtopic.php?t=62545
Try adding www. to your domain. Most often this is the problem.
I searched joomla project for vars $lang_theme_code_title... and related... There are not. Looked at Tinymce editor, it has javascript driven language file...
Most likely problem is cause by JavaScript. Try using different browser (firefox or chrome).
I read info on some other forums, some claim that temporarily disabling the antivirus solved the problem. I don't think that this will work, but you can try.
If none of this helps, you will need to give more information. Load firefox with firebug and see the NET tab for JavaScript files and Ajax Requests, most likely you'll find something interesting there (i think 500 Error)