There could be many reasons why a XML is showing a blank page.
First of all make sure that in the Post Type tab the fields you want to include in the sitemap are checked, if none is check then check them as per your need, or check them all just to test them.
If this is not the reason of the issue, then please follow the methods below.
If you still face the same issue contact our support team support@siteseo.io, we will help you out.