I found out the solution myself.
Since "ca gen rsa key 1024" is a global configuration command, you have to be in #PIX(config) mode to be able to execute this command.
After generating the RSA key-pair I did a "ca save all".
SSH-userID is always "pix"(without quotes) on pix firewalls.
SSH-userID's password is equal to the former telnet password.