[TYPO3-shop] XML Error while changing size (AJAX)

[Uli Linn]

In case anyone else has this problem:
Franz posted the following on jambage.com - it solved my problem:

Change line 225 in 
tt_products/model/field/class.tx_ttproducts_field_price.php:

$paymentshippingObj = 
&t3lib_div::getUserObj('&tx_ttproducts_paymentshipping');
if (isset($paymentshippingObj) && is_object($paymentshippingObj)) {
$taxFromShipping = $paymentshippingObj->getReplaceTaxPercentage(); // if 
set then this has a tax which will override the tax of the products
}