Thanks for your answer @bcworkz. You are right, i use a render_callbacks for my blocks. But i also save the InnerBlocks, thats why i dont understand that the_excerpt cant “see” the content:
export default function save() {
return <InnerBlocks.Content />
}
I now solved the problem like this:
$excerpt = wp_trim_excerpt($post->post_content);
$clean_excerpt = wp_strip_all_tags($excerpt);
echo substr($clean_excerpt, 0, 200).'...';
Any improvement tips are welcome!