TYPO3 Core

When going in template > Constnt editor, no editing of constants is possible.
Symptoms: no scrollbar, no event when clicking on edit icons of constants. (see screenshot.)
It seems to be a Javascript error "Class is not defined" in xxx/typo3/js/constantEditor.js in line 35

I tested my backend without passing thru my reverseproxy but it still not working.
reproduced on IE7, Firefox 3, Google Chrome
I found no work arround but the constant editor works on the same server on another instance of typo3 (v 4.1.5). same php version, same network configuration.
Any idea?
(issue imported from #M9952)