This topic is solved, but it revealed one minor issue’ish of my setup. I’ll explain it if someone will search something similar someday.
I had one VPS where Nginx takes care of SSL and some other stuff of all of my sites (plenty of ofs, english is one very strange language 
). Nginx sends requests to Varnish. It is useless reverse proxy for Discouse, but it does some filtering. Varnish sends requests to another VPS where Discourse lives using port 83. Both VPS’ listen same port and It is allowed to both IPs only. I was totally happy — until now.
I tried what happends when I used port 443 curl -I https://forum.example.tld:443. It worked nicely, because I have still valid SSL certificate on side of Discourse (I did this change some weeks ago).
@supermathie’s answer explains why this happpends and how to fix it. Sure, there is no security issues because of that what so ever AFAIK, but it is really annoying 