- SSL-certificate is purchased, approved and installed
- https://domain.tld/ seems to work
- "Server type" in Tools --> Configuration --> Site is set to "https"
- I have upgraded MODX Evolution (from 1.0.15 to 1.0.15) after SSL-certificate is purchased, approved and installed.
The problem was that the web hosting happens to use nginx, a reverse proxy or load balancer. (Also, FastCGI is not used in the case). So MODX Evolution can not get $_SERVER['HTTPS']. The solution is to employ setenvif: https://eurohoster.org/blog/administration/full/120 .