Repeatedly errors were appearing in my php error log, like this:

[15-Oct-2020 22:26:00] WARNING: [pool example.com] child 3338 said into stderr: "NOTICE: PHP message: PHP Warning: session_start(): Cannot start session when headers already sent in /var/www/vhosts/example.com/httpdocs/wp-content/plugins/whmcs-bridge-sso/includes/hooks.inc.php on line 0"

[15-Oct-2020 22:26:00] WARNING: [pool example.com] child 3338 said into stderr: "NOTICE: PHP message: PHP Warning: session_start(): Cannot start session when headers already sent in /var/www/vhosts/example.com/httpdocs/wp-content/plugins/whmcs-bridge/bridge.init.php on line 973"

[15-Oct-2020 22:26:00] WARNING: [pool example.com] child 3338 said into stderr: "NOTICE: PHP message: PHP Warning: Cannot modify header information - headers already sent in /var/www/vhosts/example.com/httpdocs/wp-content/plugins/zero-spam/classes/class-wpzerospam.php on line 938"

Normally such errors are caused when visitors (or admins) access the site, and some plugin is causing output through white space at start of file. So at first I thought that there may be some log buffering going on somewhere, as the error messages did not correspond to my visiting the site. But finally I traced it to this plugin: Using “WP Crontrol” I manually started some of the “bdt” cron jobs – and zack there were the error messages in the log. Every time.

Could you look into this please and clean it up?

Thanks, Tim