Sitemap: fatal error

On different servers, I have issues with the sitemap creation. I end up having a critical error:

AH01071: Got error 'PHP message: PHP Fatal error: Uncaught TypeError: method_exists(): Argument #1 ($object_or_class) must be of type object|string, null given in /var/www/vhosts/example.de/httpdocs/wp-content/plugins/wp-seopress/inc/functions/sitemap/template-xml-sitemaps.php:360\nStack trace:\n#0 /var/www/vhosts/example.de/httpdocs/wp-content/plugins/wp-seopress/inc/functions/sitemap/template-xml-sitemaps.php(360): method_exists()\n#1 /var/www/vhosts/example.de/httpdocs/wp-content/plugins/wp-seopress/inc/functions/sitemap/template-xml-sitemaps.php(541): seopress_xml_sitemap_index()\n#2 /var/www/vhosts/example.de/httpdocs/wp-content/plugins/wp-seopress/src/Actions/Sitemap/Render.php(71): include('...')\n#3 /var/www/vhosts/example.de/httpdocs/wp-includes/class-wp-hook.php(308): SEOPress\\Actions\\Sitemap\\Render->render()\n#4 /var/www/vhosts/example.de/httpdocs/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters()\n#5 /var/www/vhosts/example.de/httpdocs/wp-includes/plugin.php(565): WP_Hook->do_action()\n#6 /var/www/vhosts/example.de/ht...', referer: https://www.example.de/wp-admin/admin.php?page=seopress-xml-sitemap

Whenever I switch back to PHP 7.4 it works. These errors only occur on 8.0 and above.