TL;DR

This time I present a screenshot of Medium Sign Out error.

What could I conclude from this error as a Medium user?

The 504 Gateway Timeout error is an HTTP status code that means that one server did not receive a timely response from another server that it was accessing while attempting to load the web page or fill another request by the browser [source].

So Medium decided to hide HTTP status code 504.

What is Ray ID?

It is a UID that can be used by the website operator (and Cloudflare support) to potentially debug issues [source].

We found out that Medium is using Cloudflare. What is Cloudflare?

Cloudflare was created to ease these difficulties and empower users with the resources to make their sites, apps, and blogs safe and performant. This is done through the use of a powerful edge network that provides content and other services as close to you as possible, so you get the information as fast as possible [source].

The error also contains my computer IP address and HTTP error code but named as the error reference number. Cloudflare has servers in Zagreb.

What We Learned

Medium is using Cloudflare servers as intermediate servers between Medium and me. This helps Medium to mitigate DDOS attacks. Cloudflare stores my IP address. Hmm, what about other data? Is SSL broken in that Gateway relay?