Strange 521 error

[fritex]

At my first sight, without knowing your domain name, it indicates to me it could be due to SSL options at Cloudflare.

So, meaning when is for your A www and A yourdomain.com, you get 521 error?

@softadept

and also our subdomain with a nodejs server seems to encounter the same issue.

On which port does your NodeJS app running on?
Can you check compatible ports with Cloudflare and proxy your NodeJS on/over Nginx on one of the compatible ports (depending if you already have an SSL certificate at your host/origin - using HTTP or HTTP)?

https://support.cloudflare.com/hc/en-us/articles/200169156-Identifying-network-ports-compatible-with-Cloudflare-s-proxy

Community Tip - Fixing Error 521: Web server is down

Error Try the suggestions in this Community Tip to help you fix Error 521: Web server is down. Background A 521 error happens when we are unable to make a TCP connection to your origin server. Specifically, Cloudflare tried to connect to your origin server on port 80 or 443, but received a connection refused error. This is often caused by security or firewall software and happens if the origin server has directly refused Cloudflare’s proxy request. [Banner in tip catalog] Quick Fix Ideas …