@sabine: Search for the following code in image.php
<a href="<?php echo esc_url( $next_attachment_url ); ?>" title="<?php the_title_attribute(); ?>" rel="attachment"><?php
$attachment_size = apply_filters( 'catchbox_attachment_size', 848 );
echo wp_get_attachment_image( $post->ID, array( $attachment_size, 1024 ) ); // filterable image width with 1024px limit for image height.
?></a>
and replace it with
<?php
$attachment_size = apply_filters( 'catchbox_attachment_size', 848 );
echo wp_get_attachment_image( $post->ID, array( $attachment_size, 1024 ) ); // filterable image width with 1024px limit for image height.
?>