Try Checkout page in Different Browsers
First of all, try opening your same Checkout in any other browser (try the checkout page in Firefox or Internet Explorer etc.)
- If the site is not having the same issue in the another browser you know that the error is browser specific.
- If the site is having the same error, it is not browser specific error.
Make note of any browsers you are experiencing the error in. You can use this information when you are making a support request.