hi @Mario_user welcome to the forum 
as you ignored the required support template nobody can help you so far.
I’m sorry I have no idea handy to resolve the issue. I’m aware of the Office Online integration - Apps - App Store - Nextcloud app maybe you just need other integration app? wopi works same/similar for all office products ans as you recognize my name you likely found Collabora integration guide already… this is the source of truth and it should work once you complete the list.