== As an administrator, what do I have to do? ==

If your're terminating SSL traffic at load balancer level, open the server.properties ( /opt/openexchange/etc/groupware/server.properties ) and set

com.openexchange.forceHTTPS=true

you may find the old option

com.openexchange.cookie.forceHTTPS already configured, in that case just rename it to com.openexchange.forceHTTPS, thereby all links generated by the backend will suggest https is used.