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

[Uli Linn]

Hello!

I'm workting with T3 4.2.2 and tt_products 2.7. - everything worked fine 
until someone reportet that using IE the AJAX-Feature (change price 
while changing size) gives an XML Error. No Error in Firefox.

I now updated to 2.7.0.34 - and have an XML-Error in bothe, Firefox an 
IE. It says:

Error: the XML response that was returned from the server is invalid.
Received:
<br />
<b>Fatal error</b>:  Call to a member function getReplaceTaxPercentage() 
on a non-object in 
<b>/html/typo3conf/ext/tt_products/model/field/class.tx_ttproducts_field_price.php</b> 
on line <b>225</b><br />

You have whitespace in your response.

Is there any Idea, how to solve this? It's quite urgent, beacuse the 
shop is online ...

Uli