Hi,
It has to do with PHP5.3
The fix is extremely easy.
Open this PHP file:
ROOT/modules/mod_virtuemart_flexible_dropdown_cart/mod_virtuemart_flexible_dropdown_cart.php
And look at the 86th:
Find this line:
$ps_product =& new ps_product;
change it with this:
$ps_product = new ps_product;
then look at the 89th line,
find this line:
$ps_checkout =& new ps_checkout;
and change it with this:
$ps_checkout = new ps_checkout;
the problem will be gone