Nextcloud box user/password after upgrade from pi2 -> pi3

I followed successfully the instructions to migrate from Raspi2 to Raspi3.
Nextcloud is running and performing well.

When trying to use PuTTy to access my Nextcloud system the first thing I’m asked
"login as: “
and whatever i enter I’m refused access with
"Server refused our key
ubuntu@’s password:”

(In the “old days” with Raspberry 2 it was ubuntu/MyPassword)

During the setup my public key was fetched from my account with
my private key is attached in PuTTy

Any help?
Thank you