I did one test button, and copied and pasted it, then tried to test it, and received the same error.
error: <?xml version="1.0" encoding="UTF-8" ?>
> - <error xmlns="http://checkout.google.com/schema/2" serial-number="fe810e7b-e4ef-469a-9d94-d628d5a3f899">
> <error-message>Internal error in server</error-message>
> </error>