Getting system Error while Adding product into the cart -- Magento

Question

Customer gets System Error while adding a product into the cart. I think thats the session issue because when customer logs out and signs back No more "system error" appears on browser. Here is the screenshot: http://screencast.com/t/7HmL1rtQvvs

this issue only happens after using a specific payment method.

Note: we are using Magento

Thank you all

Answer 1

Does it happen after adding a product to the cart or selecting a payment method?