Hello,
We apologize for the issue you are facing with the white page in your sitemap index. Since you can see the sitemap code when checking the source, it indicates that the sitemap is being generated but not displayed correctly on the front end.
Here are a few steps you can take to troubleshoot this issue:
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Flush the Sitemaps Cache: Sometimes, caching plugins can interfere with how sitemaps are displayed. Make sure to flush the cache and exclude the sitemap from caching if you are using a caching plugin. You can find more information on how to do this here.
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Check for Plugin Conflicts: Use the Health Check plugin to identify if there is a conflict with another plugin or theme that might be causing this issue.
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Static File Conflict: Ensure that there is no static file named sitemap_index.xml on your server that could be conflicting with the dynamically generated sitemap. You can check this via FTP.
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Server Configuration: If you are using Apache or Nginx, ensure that the appropriate rewrite rules are set up in your .htaccess or Nginx configuration file, respectively. You can find the necessary rules for Apache here and for Nginx here.
If after trying these steps the issue persists, please reply to this ticket, and a human agent from the Rank Math Support team will take over and assist you further.
Thank you for bringing this concern to our attention!
Sources:
Rank Math Sitemap Issues and Their Fixes » Rank Math
Hello,
Thank you for contacting us and sorry for any inconvenience that might have been caused due to that.
This issue is occurring because on your database entries, there are some Unicode characters that are preventing the sitemap from correctly loading.
Please access your WordPress database and check the table wp_options. You’ll notice entries like blogname with Unicode characters instead of the correct characters in there.
To be more specific, you may take a look if you find a similar entry like Mokotów in the option_value column. Here is an example below for your reference:

Please try to replace this entry with Mokotów, and then the sitemap should start working fine again.
Let us know how it goes. Looking forward to helping you.
Thank you.
Thank you very much for your help, I contacted the web host as well and described the problem. They helped to solve it. They changed the word to “Mokotów” as you suggested, then I reinstalled the plugin and the sitemap is back in place 🙂
Hello,
We are super happy that this resolved your issue. If you have any other questions in the future, know that we are here to help you.
If you don’t mind me asking, could you please leave us a review (if you haven’t already) on https://wordpress.org/support/plugin/seo-by-rank-math/reviews/#new-post about your overall experience with Rank Math? We appreciate your time and patience.
If you do have another question in the future, please feel free to create a new forum topic, and it will be our pleasure to assist you again.
Thank you.