Error: Cannot modify header information - headers already sent by

Hello, I run the finished website from a sucuri.net check, and i get these results for some pages:

<b>Warning</b>: Cannot modify header information - headers already sent by (output started at /home/kefalas/public_html/wp-content/plugins/wp-fastest-cache/inc/cache.php:358) in <b>/home/kefalas/public_html/wp-content/plugins/super-forms/includes/sessions/class-wp-session.php</b> on line <b>128</b><br />

https://sitecheck.sucuri.net/results/kefalas.gr

Any idea how to fix that? 

I don't see this issue on your site its loading fine. Which URL Do you see this problem in.