First, sorry for my english, now go to the solution:
I had the same problem, but I have the solution. The problem is when the servers run MySql 5, on MySql 4.xx works perfect.
Well, you must add this code at line 87 in the quick_stockupdate.php file:
$stock_update=$_POST['stock_update'];
The result must be something like this:
<?php //Line 86
$stock_update=$_POST['stock_update']; //Line 87
if ($HTTP_POST_VARS['stock_update']) { // Line 88
// Line 89
while (list($key, $items) = each($stock_update)) { //Line 90 -- stupid line>:( ouch!!.
// update the quantity in stock // Line 91
etc...
If you want download the contribution fixed I upload an update here: http://addons.oscommerce.com/info/3542
Bye