Because I wasn’t able to see a virtualhost in the gui, I manually added a new one with forced SSL and requested a new LE-cert. To make sure, I executed above commands again.
When I go to collabora.domain.tld, I am presented with the default Nethserver page.
Collabora works in Nextcloud, you have to install Nextcloud to work with it, see the docs.
Without a specific path like in the Collabora admin page (https://collabora.yourdomain.com/loleaflet/dist/admin/admin.html) you get the Nethserver default page. That’s ok.