It is really a strange issue. Can you please let me know when this issue started appearing - is this after an upgrade. Do you have a custom checkout widget. Have you customized or extended the built-in Checkout widget.
In addition, can you please try to create a new page based on a default page template, and then drag and configure the default checkout widget and let me know if the issue still persists.
Furthermore, can you please delete the error logs under the ~/App_Data/Sitefinity/Logs
folder, then try to reproduce the issue again and send me the files generated under the above mentioned folder. This might give us additional information on what causes the issue.
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