Solved it by using the
XFCE
Armbian_25.2.1_Uefi-arm64_bookworm_current_6.12.13_xfce_desktop.img.xz
instead of the
cinnamon
Armbian_25.2.1_Uefi-arm64_bookworm_current_6.12.13_cinnamon-backported-mesa_desktop.img.xz
edition.
I think it might need a different nextcloud client for both editions.
If one is using the authentication via browser and the other needs Gnome online accounts.
