Featured image Media Upload not showing in posts/pages

Since I upgraded to WordPress 6.1.1 the option to upload featured images in my posts and pages disappeared. I cannot edit the existing ones and I cannot upload new images. Usually it was an option in the right side panel.

In my Theme I activated featured images like this (functions.php):

if (!function_exists('ksv_setup')) {
function ksv_setup() {
add_action('after_setup_theme', 'ksv_setup');

  What can I do?


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