getting error when load testing

I am trying to load test my app with JMeter. When I run just one thread, it works fine. But as soon as I add another thread I get an error 100% of the time. The error is a server error 500 (internal error). How do I go about debugging this?

proof that an old dog can learn new tricks

What web server do you use?

ci-phpunit-test | CodeIgniter Testing Guide | CodeIgniter 3 to 4 Upgrade Helper

I am using Apache on AWS. I should add that I use RDS and that the report generation function is heavily RDS based. I did some more testing last night and found that if I start up 3 threads, but have a ramp time of 35 seconds between startups, it seems to work fine. It is almost as if only one thread can run at a time. 35sec/3 threads is 11.67second delay, which is just about enough to allow the previous thread to finish. 

Can I assume that there is more than one pipe from my EC2 server to RDS? It almost seems like there is only one.

proof that an old dog can learn new tricks