Hello!
Thanks for getting back. I was able to replicate the issue on my end.
First of all, can you please make sure you’re running the latest version of WordPress (5.7.1), Yoast SEO (16.2), as well as the latest version of your theme and ALL other plugins? Please also check that your site meets our plugin requirements.
If everything’s updated on your end and your site meets our plugin requirements, we need you to perform a conflict check.
The fastest way to do this is to?deactivate all non-Yoast plugins and switch to a standard theme?like?Twenty Nineteen.?So only the Yoast plugin is activated together with the Twenty-Twenty theme. Then load your sitemap. Does the issue occur? If not, reactivate your other plugins one at a time and check if the issue occurs again for every plugin reactivated. Lastly, switch to your theme.
Test this on your development or staging site, if you have one. If not, we recommend using the?Health Check & Troubleshooting?plugin. This plugin allows you to run a conflict check?without affecting normal visitors to your site.
As we can imagine that you’re unfamiliar with checking for conflicts, we’d like to point you to a step-by-step guide that will walk you through the process:?How to check for plugin conflicts.
If you do not feel comfortable to do this yourself or if this does not solve your issue, our Yoast SEO Premium plugin comes with one year of (technical) support.`