I have a wordpress 5.1.1 site with rokbox and rokgallery installed. There is a conflict when both plugins are activated at the same time.
Rokbox Version 2.52
RokCommon Version 3.2.6
RokGallery Version 2.45
PHP Version 7.3.4
In the error log I read the following:
PHP Warning: session_start(): Cannot start session when headers already sent in /home/accessi2/wp-content/plugins/wp_rokcommon/RokCommon/Session.php on line 82
PHP Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /home/accessi2/wp-includes/class.wp-styles.php:95) in /home/accessi2/wp-content/plugins/wp_rokcommon/RokCommon/Session.php on line 82
I have replicated the same behaviour on an empty fresh wordpress installation with no other plugins installed other than the 3 mentioned.
Are you able to test on PHP 7.2 and see if the error is still present?
If you're using a Gantry 5 theme I'd recommend using the built in Lightcase Atom which works nearly identically to the way RokBox works; which then may resolve your conflict.
PHP Warnings are generally not service affecting. Unless your actively troubleshooting something it's best practice to restrict your PHP error log to critical events only so that non-service affecting errors don't fill up the log.