SSH Private Key for Minion Bootstrap

[ipanmeri]

Hi, in my new uyuni installation I am at the point to bootstrap the first client.
I would like to bootstrap a sles12sp5 using SSH Keys.
I have generated a new rsa 2 keypair with puttygen on Windows, put the public part in /root/.ssh/authorized_keys of the target host and use the ppk (private part) in the Uyuni Web GUI.
I can use this keypair with success to log into the target via putty.

But on the GUI I get this msg:
stderr: "Load key "(...omissis...)088eda1ab8e8": invalid format

Can't find any hint in the doc about how this ssh key pair must be configured, what type is allowed or supported.
Can anyone kindly point me to the documentation for this?
I can of course enable root login for just the registration, but would really know how to use ssh keys.
Thanks a lot

[ipanmeri]

I tried the /var/lib/salt keys and looks somewhat better ... at least i get another error.