I’ve just checked all posts. They are set to Standard. I created a new post for test purposes. No luck, the blog page shows no title and the full text instead of the excerpt.
I temporarily deactivated the functions.php in my child theme. Made no difference. If I change the content.php (line 67) from the_content() to the_excerpt() the blog page changes and shows the excerpt. If I add
<?php the_title( '<h3>', '</h3>' ); ?>
right before the the content on line 66, the title is shown on the blog page. If I add it to the header section (line 45) it is NOT shown on the blog page.