Hi @ZA-S,
4.4 is very well supported (especially since its still based on the same Debian release as 4.3), its just that the extra code in the integration package does not yet recognise it. There is a commit already that adds it, but I am waiting on 8.7.1 to push an update for the app.
Work around would be manually configuring the repository in /etc/apt/sources.list (but don’t forget to remove it afterwards). this can be done like this:
echo "deb https://$(ucr get kopano/repo/username):$(ucr get kopano/repo/password)@download.kopano.io/supported/core:/final/Univention_4.3/ /" >> /etc/apt/sources.list
Alternatively you could apply the changes from https://stash.z-hub.io/projects/K4U/repos/kopano4ucs/commits/c3a6bbb86fbc011ac3f40ac1a579b053ae92019d#conffiles/etc/apt/sources.list.d/kopano-core.list to you local files. These files are stored in /etc/univention/templates/files/etc/apt/sources.list.d.