Page freezes when I clic "add to cart"

-- HikaShop version -- : 2.3.5
-- Joomla version -- : 2.5.28

Hi,
My website was working perfectly well until very recently.
The issue is the following: when I try to add a product to the cart by clicking on the "add to cart" button, nothing happens. The page just simply freezes, and I cannot even relog into the website.
I have to close google chrome and re open it before I can have access to the website again.
I don't know whether there is any connection, but I upgraded my joomla version to 2.5.28 on 19th Jan 2015.
Could you have a look and let me know.
These bugs happen as well on the French as on the English pages. On any page in fact...

Thanks in advance for your help.

Best Regards,
Vinc

Hi,

Thank you for your help.

Here is the error message :

Strict Standards: Declaration of JParameter::loadSetupFile() should be compatible with JRegistry::loadSetupFile() in /home/herballe/www/libraries/joomla/html/parameter.php on line 512

Fatal error: Cannot pass parameter 1 by reference in /home/herballe/www/plugins/system/custom_quantity_tax/custom_quantity_tax.php on line 110

Thank you very much. It's working now!