The total cart amount does not update automatically upon clicking "Choose a Delivery Method" and is showing a Server Error upon clicking "Pay Now"

The process works like this:
1. The user adds to the cart a product.

2. Then he will click the selected Delivery Method.
3. Then click "Pay with (depending on the chosen payment provider)".

4. Finally click the "Pay now" button.

This error occurs.
Server Error
We are not able to process your payment.
The cart has been updated. Please refresh the page.

Have you experienced this already? Any advise? Thanks a lot.