Can not save changes in Performance

Thanks for quick reply

Thank you for your help.

I had the plugins before, too.
if I turn off the caching plugin, not much changes.

cmb2 redux-framework
wordpress-seo-premium
wp-speed-of-light
contact-form-7
regenerate-thumbnails wp-instagram-widget
wp-speed-of-light-addon
facebook-for-woocommerce
remove-yoast-seo-comments
wpseo-local
yith-woocommerce-wishlist
revslider
wpseo-local-woocommerce
yoast-seo-search-index-purge
js_composer
woocommerce
wpseo-news
mailchimp-for-wp
woocommerce-gateway-paypal-express-checkout
wpseo-video
Nginx-FastCGI-Cache-master
woodmart-core
wpseo-woocommerce

DATA IN PRIVATE
Debug on. If you want can give you plesk too!

FIXED
Warning: DOMDocument::loadHTML(): htmlParseEntityRef: no name in Entity, line: 13 in
/var/www/vhosts/cosmeticbag.de/httpdocs/wp-content/themes/woodmart-child/functions.php
on line
95

   // Get page content
    $html = new DomDocument;
    $html->loadHTML( $content );
    $html->preserveWhiteSpace = false;

His is just a warning, not an error. Your script will still process it.
To suppress the warnings set

CHANGE WITH ` // Get page content
libxml_use_internal_errors(TRUE);
$html = new DomDocument;
$html->loadHTML($content);
$html->preserveWhiteSpace = false;`