ReadOnlyUser
I gave a short try to Celeste, but this was very buggy. Not usable!
Nextcloud support highlighted that the Flatpak of their client is NOT official/verified, but they are in contact with the maintainers of that flatpak and confirmed that official sources are used to build it.
I didn't managed to get it properly running at Fedora/Gnome. After switching to Fedora/KDE for other reasons, the Nextcloud Flatpak works fine now.