Install DevFormatter, and activate it, you should found the whole editor has been broken. Then FTP to your website, download and modify the file in /wp-content/plugins/devformatter/ with the name of "devinterface.php".
On line 465, you should see these code:
for(i = 0; i < edButtons.length; i++)
if(edButtons[i].id == 'ed_code')
edCodeId = i;
$(\"input#ed_code\")[0].onclick = function(){
ed_codeInsert(edCanvas, edCodeId);
};
Remove them, save and upload this file to your website, you will find the editor has been recovered.
WARNING: This workaround would remove the function of the link to DevFormatter of "code" button. You have to insert the "code" tag by yourself, for example, <code lang="c"> or something similar.