Does not work with Splash Theme from my ThemeShop

O.K., actually it seems to work. It was a problem with a thirt-party-plugin.

But now I have another problem. The generated featured image is not add automatically inside the post content so it is not shown on single-post view. I have to add each image by hand.
Is there a way or another plugin that will add the generated featured image automatically inside each post?

I’ve always allowed the theme to handle where the featured image is shown. If your theme has a single.php file, you can put this code where you would like the featured image shown.

<?php
if ( has_post_thumbnail() ) { // check if the post has a Post Thumbnail assigned to it.
  the_post_thumbnail();
}
?>

Thanks Chris, I will try this.

But I have to come back to my first question. Plugin seems not to work with Splash Theme from myThemeShop. No Image is getting generated after publishing a post.

Sometimes it works, sometimes not.

Any idea to fix the issue?

Turn on error reporting in your wp_config.php file and let me know if any errors are being generated. If there are any, please copy and paste them here.