Thank you Reinis for the tip with the outdated version of Nginx, now I am using 1.12 ;)

But the problem still remains and it is getting weirder with every minute.

Even if I disable the 3jgkp.conf completely so only one server block remains, armapedia.de will be redirected to armapedia.3jgkp.de. Maybe that is because of the cookie you mentioned earlier, but I have no clue how to delete this information or where this magic redirection is coming from. Can this be a problem of my DNS configuration? Otherwise, I ran out of any option left to try or to look into...