Ok, get it done by myself, opened the sd-card in an other linux system:
First removed the password check in /etc/passwd from
INI:
root:x:0:0:root:/root:/bin/bash
to
INI:
root::0:0:root:/root:/bin/bash
then in /etc/ssh/sshd_config (only the changed lines)
INI:
Port 22
PermitRootLogin yes
#PubkeyAuthentication yes
PasswordAuthentication yes
PermitEmptyPasswords yes
UsePAM no
#AllowUsers solar-assistant
after that just boot and login with ssh as root, no password check, then you can change password for root to something else:
Bash:
root@machine# passwd
New password: [desired password]
Retype new password: [desired password]
Now you have a password for root that you know. No change in etc/shadow needed, but you don't even need that, just put back the "x" in /etc/passwd and everything is as before.