As of version 3.14.0, neoCatalog could return an "_Internal error" message at Shopping Cart checkout.
The reason is that the "Cart" request type, which is automatically created when using the shopping cart for the first time, had been deleted by the Administrator and this has been logged in neoCatalog ever since, the time during which the checkout was performed.
To solve it, the Administrator has to log out and back in so that the "Cart" request type is available again.