Problem solved! A change in the style.css of the theme caused the error:
#access a {
color: #777;
display: block;
float: left;
font-size: 14px;
text-transform: uppercase;
padding: 16px 0 0; => I changed 16px to 10px (because of another problem with IE)
height: 39px;
}
After changing the value back to 16px the submenu appears regulary.
