Ssh access not configurable / working

i had to factory-reset my nextbox after some month of offline/inactivity.

i could do the initial setup via the the webapp but when i try to configure
nextbox / settings / ssh access
and put various (valid) ssh public keys in; I only get a pw prompt b/c the public key configuration somehow does not work.
i tried (openssh) public keys in id_25519 and rsa format.

i am currently in an offline setup;
connected to the nexbox via a crossover ethernet cable or a simple switch.
so my ssh login command is like:
“ssh -v -i ~/.ssh/ts nextuser@

part of ssh-output of “ssh -v -i ~/.ssh/id_rsa nextuser”

debug1: Offering public key: /Users/ts/.ssh/id_rsa RSA SHA256:E/7pyCemiW9iS8U4anck5KdtUzMQXy9/nQKI6BZNVqo explicit
debug1: Authentications that can continue: publickey,password
debug1: Next authentication method: password
nextuser’s password:

as said, http works, admin user is created, recommended nextcloud apps are installed but i cannot (re-) configure ssh access.
I need this to mount my backup disks to be able to restore my configuration.

any help / ideas are very much appreciated.