Intermittent timeouts and 500 errors with Chef server 12.1.2

I’ve switched from Chef 11 to Chef 12 server (hosted on AWS) about 6 months ago and after a certain period of time I started experiencing occasional timeouts and now I’m getting both timeouts and error 500s on a regular basis. I’m trying to figure out how to troubleshoot the problem and I can see 500s in NGINX log but nothing else that would point me in the right direction.