This issue is related with the basic store configuration. It may happen mostly when you successfully configured an SSL connection in your store. If behavior described below is similar to your case, follow the article, at the end we will show you how to fix it.
If your customers have products in the shopping cart and after pressing the Checkout button in the mobile app cart, application will notify them that shopping cart is empty, it means that you should check your store configuration. Below you will find a little example from our demo mobile application:
Please make sure that your store configuration doesn't have common mistakes, like wrong store URL in Store settings or what's mentioned at the beginning - if you force to use any type of www prefix, check both URL addresses - Store URL and URL in the mobile app configuration are the same, so contain www prefix or not. Please note that we recommend to use the "Doesn't matter" setting for "www prefix requirements", for us it the most secure configuration.