Error In Pro Sites

[robvideo]

How Do I Solve this one please
Cannot start session when headers already sent in /home/ffsdsea/public_html/wp-content/plugins/pro-sites/pro-sites-files/lib/ProSites/Helper/Session.php on line 29

[mrbm]

I would love to know the answer to this too. Wonder if WPMUDEV will maybe follow up on this..i realize its a sunsetted plugin but its been occurring prior to sunset and was never resolved: https://wpmudev.com/forums/topic/pro-sites-php-warning-cannot-start-session-when-headers-already-sent-in-prosites-helper-sessio/

In the situation I have been tracking, prior to that error I see:
PHP Warning: Cannot modify header information - headers already sent in /srv/users/XXX/apps/wordpress/public/wp-includes/functions.php on line 6865

Which leads to the wordpress setting:

function send_frame_options_header() {
        header( 'X-Frame-Options: SAMEORIGIN' );
}