Error message: 

Cause:

OneStepCheckout is not configured correctly - The error is due to the shipping method not having a valid postal code

Solution:

This is solved by filling in a default postal code in the configuration of OneStepCheckout.

Please note that if the client specifies a different postal code, it will always overwrite the default one.

Go to System > Configuration > Sales > OneStepCheckout > General

And enter a valid Belgian postal code: