Hello,
This issue is possibly occurring because, on your database entries there are some Unicode characters that are preventing the sitemap from correctly load.
Please access your WordPress database and check in the table wp_options. You’ll notice entries like blogname with Unicode characters instead of the correct Vietnamese characters in there.
To be more specific, you may take a look if you find similar entry like Áo thun Tees in the option_value column. Here is an example screenshot below for your reference:

Please fix that entry or any other similar type of entry by removing Unicode characters. You can also just replace the entry with Áo thun Tees and the sitemap should start showing up fine.
If you see your sitemap still loading with the same error even after applying the fix, please try clearing the caches from your site and it should work fine.
We could also perform these checks. But for that, we would need cPanel/phpMyAdmin access to handle the situation.
Let us know how it goes. Please do not hesitate to let us know if you need our assistance with anything else.
Thank you.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.