I noticed this issue with wpForo in my error logs:
WARNING: [pool smallcaps.com.au] child 1962 said into stderr: "NOTICE: PHP message: PHP Warning: session_start(): Cannot start session when headers already sent in /var/www/vhosts/smallcaps.com.au/httpdocs/wp-content/plugins/wpforo/wpf-includes/class-notices.php on line 16"
How can this be fixed? Does your plugin need updating?
This error says that wpForo doesn't have any PHP error. Your output is affected by other plugin error and it generates " headers already sent " NOTICE in wpForo functions. It'll generates such notices in all plugins who have session functions. So please find the plugin who generates errors or outputs in header. Deactivate those one by one and check, once the wpForo Notice is gone then the problem is fixed and you found the problem maker plugin.
This error also can be from functions.php of your theme.
Another explanation: https://wordpress.org/support/topic/php-warning-in-cron-daemon/#post-12238846
Also this error could mean some spaces (white space) you added IF you edited some php file in the core or a plugin. If you haven't touched any code, then ignore my reply.