The first thing you need to do is determine whether you have a blank page or a white page. It sounds all the same but there is a difference. Check the source code of the blank/white page.
If the source code of the sitemap page is empty, there must be a conflict with another plugin or the theme. You can use the Health Check plugin to determine which plugin or theme is causing this. Another scenario is that you have a static file with the same name as you sitemap file (sitemap_index.xml). You should FTP into your server and remove the file.
If that doesn’t work, we might need to take a closer look at the settings. Please edit the first post on this ticket and include your WP logins in the designated Sensitive Data section.
It is completely secure and only our support staff has access to that section. If you want, you can use the below plugin to generate a temporary login URL to your website and share that with us instead: