Why is my Open OnDemand RStudio Server session giving me a 502 or 503 error?

When I try to launch my Open OnDemand interactive RStudio Session job, I am taken to a page with an HTTP Error 502, or 503. Why does this happen and what can be done about it?

Please refer to this other Ask.ci question: Why do I get a gray screen after launching my Open OnDemand RStudio Server session?