I just upgraded to WordPress 5.7 and I don’t see the option to add a post thumbnail any more.

I checked my functions.php and everything is still fine:

add_theme_support( 'post-thumbnails' );

I have tried all kind of stuff, but nothing that solves the problem, where it used to work perfectly before the upgrade.

I also tried:

function theme_setup(){
    /** post thumbnail **/
    add_theme_support( 'post-thumbnails' );
}
add_action('after_setup_theme','theme_setup');

Any ideas?