No you are wrong, this is not an issue due to plugins and most importantly the sitemap is perfectly valid and well formatted XML: https://www.arfu.nl/index.php?option=com_jmap&view=sitemap&format=xml

The problem arises only when opening the styled format, indeed it looks like your server is not able to open XSL files, maybe a permissions issue, htaccess issue, etc but it throws 403 permission denied when opening the XSL file: https://www.arfu.nl/components/com_jmap/xslt/xml-sitemap.xsl

Check your htaccess or try to report this to the hosting provider.

Best regards
John D.