Testing Mollie gateway and get error

I was testing out the Mollie payment gateway with MarketPress, when trying to checkout choosing for example ccard or PayPal I get the next page to finalize payment but after this page I get this 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 /public/sites/shop.scrapplezier.nl/wp-content/plugins/marketpress/includes/common/payment-gateways/mollie.php on line 199

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