Here is the solution:
Find function parallaxcontentslider_shortcode_call
and then instead of this line:
get_wp_parallax_content_slider2( $categ );
insert this bunch of code:
ob_start();
get_wp_parallax_content_slider2( $categ );
$output_string=ob_get_contents();
ob_end_clean();
return $output_string;
Cheers