Fatal error from Mollie confirm order

Hi folks, I am getting this error report when testing the payment gateway on a sub site as we are using sub sites for different courses. the site in question is recht.oursite dot nl

I have activated access and switched the sites to English.

Error.....

Fatal error: MP_Gateway_Mollie::process_confirm_order(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "MP_Cart" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in
/wp-content/plugins/marketpress/includes/common/payment-gateways/mollie.php on line 199

Any quick fixes for this?

Cheers.

John