I've been trying to test my hosted checkout page in sandbox env, But this error seems to be persistent with any card I use provided by you guys on the test card page. So when I tried to see what requests are going when I submit the form. I saw two requests, One for token which seems to be failing due to cors issue says its the "Internal server Error" and the second requests gives a bad request.
Kindly refer to these images, let me know if you have any questions.