We set the timeouts of nginx and jetty server (65 seconds )to be more than what we have at ELB (default 60 seconds).
Still I see that issues persist. I had already raised ticket when we saw ELB_5XX. After reading your blog post, I had asked them to disable pre-open optimisation. But tech support replied saying there is no such configurability he is aware of and he escalated to service team. I hope to receive positive feedback.
And when you said new ELB’s, Were you referring to Application load balancer ?