The problem is the links are in the form:
a href="offerta_didattica/index.htm" onMouseOver="return lightUp2('elMenu1',
I1ImageOn, event)" onMouseOut="return dimDown2('elMenu1',
'stdoc/offerta_did1.gif')">...
stdoc/offerta_did1.gif does exist in the mirror.
The site's javascript (menu_array.js) has the following code:
 Menu1=new Array("rollover:/stdoc/offerta_did1.gif:/stdoc/offerta_did2.gi...
Those are absolute references which don't exist in a mirror.