I am not able to use https on the KLayout.de. I do not know how this server ist hosted, however there is a project that allows to genereate valid cetrificates for https for free. These certificates are accepted by all major bowsers.
If the installation would be running on Plesk this can even be installed as a module.
As we log in with a password on the forums it would be much safer, if https would be active.