blank page after checkout

-- HikaShop version -- : 2.3.4 Business
-- Joomla version -- : 2.5.27
-- PHP version -- : 5.3.29
-- Browser(s) name and version -- : Chrome 38.0.2125.111
-- Error-message(debug-mod must be tuned on) -- : blank screen after checkout

pls help me to get the error fixed.
It was really working until I have installed the newer version of Hikashop and Hikamarket and that sounds strange for me.
I allready tried to customize the paypal plugin - no effect.

Hi

here is the output of my debugging:

Notice: Undefined variable: config in /mnt/weba/b2/54/53672054/htdocs/joomla_03/administrator/components/com_hikamarket/classes/order.php on line 1612 Fatal error: Call to a member function get() on a non-object in /mnt/weba/b2/54/53672054/htdocs/joomla_03/administrator/components/com_hikamarket/classes/order.php on line 1612

line 1612:

$price_with_tax = ($config->get('calculate_vendor_price_with_tax', false));

awesome!
thank you very much - right now it's working
sunny greetings from germany
Fritz