I finally was able to make Nextcloud Office work. My installation is using the nextcloud snap on an ubuntu base (in an lxd container). The problem and partial fix are not specific to a version of nextcloud, nextcloud hub or ubuntu. I have had the issue that documents will never stop loading for a very long time.
Today I decided I’d try to run Collabora_Online.AppImage, located in /var/snap/nextcloud/current/nextcloud/extra-apps/richdocumentscode/collabora on the nextcloud server.
To my surprise, it gave a simple error:
dlopen(): error loading libfuse.so.2
I solved it using an equal simple:
apt-get install libfuse2
I then started the Appimage manually on the command line and there we go, nextcloud office is able to load documents for editing etc.
One problem that still persists, is that the Appimage is not started automatically after a reboot. If anybody can hint me to the mechanism that should start it, I think my installation is finally fully functional.