That was it! It works. Thank you so much!
The activation link still starts with “http://http://…”, could this be caused by discourse.conf in the nginx/conf.d? I copied it from the howto on Running other websites on the same machine as Discourse, only altering the server_name.
Or could this be caused by my nginx “sites-enabled”? The “location ~ .php$”-part ended up beeing a mix out of “How To Configure Nginx as a Reverse Proxy for Apache” (How To Configure Nginx as a Reverse Proxy for Apache | DigitalOcean) and “Running other websites on the same machine as Discourse” (Running other websites on the same machine as Discourse):
location ~ \.php$ { proxy_set_header X-Real-IP $remote_addr; proxy_set_header X-Forwarded-For $remote_addr; proxy_set_header Host $host; proxy_pass http://unix:/var/discourse/shared/standalone/nginx.http.sock:; proxy_set_header Host $http_host; proxy_http_version 1.1; proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for; }