Magento 2 checkout to work properly

Hi, We are experiencing issues at checkout, please see attached images and only bid if you have solved this before and can deliver a solution within 24 hours (very important). Issue as of now: Checkout is completed with a 500 internal error for credit card payment and a 400 error for paypal payments. However, the purchase goes through properly and is displayed in both Magento 2 admin and Braintree dashboard, but customer is not being redirected to thank you page nor is he receiving any order confirmation by email. We expect the checkout to: - generate the order properly in backend and in braintree/paypal - redirect customer to the default thank you page for that order - send out order confirmation for the order to customer without any errors... We use onstepcheckout, and offer credit card & Paypal through Braintree as well as bank wire as payment options. In your bid please state if you have solved this before and if you will be able to deliver within 24 hours (preferably within 12 hours).