Session Errors with Pro Sites

Here are the warnings I'm getting:
PHP Warning: session_start(): open(/var/lib/php/session/sess_pjpvdnnk9iljg3an353493o4j0, O_RDWR) failed: Permission denied (13) in /usr/local/vhosts/balloonhq.org/htdocs/wp-content/plugins/pro-sites/pro-sites-files/lib/ProSites/Helper/Session.php on line 29
Warning: session_start(): open(/var/lib/php/session/sess_pjpvdnnk9iljg3an353493o4j0, O_RDWR) failed: Permission denied (13) in /usr/local/vhosts/balloonhq.org/htdocs/wp-content/plugins/pro-sites/pro-sites-files/lib/ProSites/Helper/Session.php on line 29

These errors show up in my log files and most annoyingly, when I run batches using WP-CLI.