Error: Cannot modify header information - headers already sent by

I have run diagnostics and got:
Warning: Cannot modify header information - headers already sent by (output started at /home/httpd/vhosts/medischondernemen.nl/httpdocs/joomla/components/com_sef/sef_incl.php:0) in /home/httpd/vhosts/medischondernemen.nl/httpdocs/joomla/index.php on line 224

On the homepage (Ijoomla) certain links to content items don't work, but others do.

Any suggestion/help how to solve??

Where exactly do you get that?

First page if I turn diagnostics in Sef on


Thanks!