Thanks for replying to my last question. I gave up on WordPress on Heroku and now I’m on AWS (Ubuntu, Nginx, Varnish). Everything was working on a different instance just fine but now this error happens.
Not sure if I am using a load balancer.. maybe NginX is a load balancer out of the box?
I created a sessions/ in my wordpress install folder and tried to update wp-config.php and index.php with: session_save_path('/home/deployer/sites/ec2-54-226-138-235.compute-1.amazonaws.com/sessions/');