Ok ok ok … I found it!!! Ismael already posted the solution on 13 januar!
He wrote on woocommerce-shop-different-image-quality-depending-on-how-i-open-the-product/#post-1378523 “You can add the following code in the functions.php file to adjust the size of the catalogue images.”

add_filter( 'avf_wc_before_shop_loop_item_title_img_size', 'avf_wc_before_shop_loop_item_title_img_size_mod', 10, 1 );
function avf_wc_before_shop_loop_item_title_img_size_mod( $thumbnail_size ) {
    return 'woocommerce_thumbnail';
}

Works perfectly – problem solved.

Hi Mike,
Thanks for your quick support – I’ll have a closer look at the “Page Sidecar Navigation“. I didn’t know about it until now. Maybe I’ll need it in another place! Great – one never stop learning!
Best regards,
Lutz

Hi Ismael,
… sometimes the technology is not comprehensible. I don’t remember changing anything in the Quick CSS. I had deleted the browser cache several times. The menu in smartphone mode now works as it should again: the white area covers the whole screen. FYI: The only thing I changed is the heading below the slideshow – from uppercase to standard.
Cheers,
Lutz

Hi Rikard,
Sorry for the delay – everything is working fine again, you can close the topic. Thank you!

Best regards,
Lutz

Another problem is that the slider in the footer is repeated (!!!) (after I installed Enfold 5.0 and the latest Revolution Slider) and covers the content of the footer and socket.