I explain the problem.
On fresh install of koozali server 10.1 I have installed NEXTCLOUD. But I have strange behaviour with it. When I install calendar on NEXTCLOUD and I want to share it with other members, the GROUP and CONTACT list are empty
When I had user or Group on server back-end the user is replicated on NEXTCLOUD very well.
First thing to make clear is what version of Nextcloud are you talking about?
Next I think what you are you are asking is when you go to share a Calendar there is a box that says 'Share with other users or groups' and that it is empty.
When you click in the box it says 'List is Empty'
You have to start typing before anything shows.
It seem that is a NEXTCLOUD bug and it is very annoying.
If it is the above as I suspect then yes, it is a Nextcloud issue and you need to raise it with them. We do not patch Nextcloud.
I would imagine that there is probably already a bug open but you will have to check their bug tracker.