Nick, you are my hero.
As you said, I did rclone config with pi user when creating the chunk-remote.
And I do sudo rclone copyto with root ... so it was an user/config file problem.
Now it works like a charm, except:
My 10G file is chunked into 12 pieces now in a folder on my Nextcloud Drive.
How do I unchunk them into one 10G file directly on my Nextcloud Remote Drive?
PS: Nextcloud supposedly supports files >2G even in 32bit environments for quite some time now (I think Nextcloud "chunks" by itself when handling larger files).