We reviewed the source code of your sitemap and it appears that something is inserting the following code before the beginning of the XML document:
<script type="text/javascript">
var bhittani_plugin_kksr_js = {"nonce":"d93ca83eb8","grs":true,"ajaxurl":"https:\/\/lenkradkralle.net\/wp-admin\/admin-ajax.php","func":"kksr_ajax","msg":"Gib eine Bewertung ab!","fuelspeed":400,"thankyou":"Danke!","error_msg":"An error occurred","tooltip":"0","tooltips":[{"tip":"","color":"#ffffff"},{"tip":"","color":"#ffffff"},{"tip":"","color":"#ffffff"},{"tip":"","color":"#ffffff"},{"tip":"","color":"#ffffff"}]};
</script>
<script type="text/javascript">
var bhittani_plugin_kksr_js = {"nonce":"d93ca83eb8","grs":true,"ajaxurl":"https:\/\/lenkradkralle.net\/wp-admin\/admin-ajax.php","func":"kksr_ajax","msg":"Gib eine Bewertung ab!","fuelspeed":400,"thankyou":"Danke!","error_msg":"An error occurred","tooltip":"0","tooltips":[{"tip":"","color":"#ffffff"},{"tip":"","color":"#ffffff"},{"tip":"","color":"#ffffff"},{"tip":"","color":"#ffffff"},{"tip":"","color":"#ffffff"}]};
</script>
<script type="text/javascript">
var bhittani_plugin_kksr_js = {"nonce":"d93ca83eb8","grs":true,"ajaxurl":"https:\/\/lenkradkralle.net\/wp-admin\/admin-ajax.php","func":"kksr_ajax","msg":"Gib eine Bewertung ab!","fuelspeed":400,"thankyou":"Danke!","error_msg":"An error occurred","tooltip":"0","tooltips":[{"tip":"","color":"#ffffff"},{"tip":"","color":"#ffffff"},{"tip":"","color":"#ffffff"},{"tip":"","color":"#ffffff"},{"tip":"","color":"#ffffff"}]};
</script>
<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="//lenkradkralle.net/wp-content/plugins/wordpress-seo/css/main-sitemap.xsl"?>
<urlset xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance" xmlns:image="https://www.google.com/schemas/sitemap-image/1.1" xsi:schemaLocation="https://www.sitemaps.org/schemas/sitemap/0.9 https://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd https://www.google.com/schemas/sitemap-image/1.1 https://www.google.com/schemas/sitemap-image/1.1/sitemap-image.xsd" xmlns="https://www.sitemaps.org/schemas/sitemap/0.9">
We would recommend performing a conflict check to check whether this additional code is being inserted by another plugin or theme. We have additional information on how to check for a conflict here: https://kb.yoast.com/kb/how-to-check-for-plugin-conflicts/