Since a few time Joomla menu in backend administrator is displaying without a decorator.
No errors are thrown by PHP, so I can't understand why is doing this way.
Ok I digged in
Inside the #_menu table , there was a voic with a 'level' value very high like 4559099066767 .
The 'level' voice, is the level of nesting of category. That causes an infinite loop inside the code, giving the result as the screen.
My version was 2.5.9
Thanks