To be honest, my provider already enabled this months/years ago for the root domain and the www-sub-domain, but for other sub domain you needed to create a support ticket, what I finally did.
Unfortunately it also broke my site… -_-
The strange thing was that the website worked as expected as soon as I logged in, so everything pointed to a cache issue, but I wasn’t able to find a cache setting. Eventualy after a small hours searching. I discovered that I had an disabled plugin called Hyper Cache that, apparently, still cached a part of my theme, causing the site to break. Removing the plugin fixed it eventually. So yeah!!