thank you for this hint. But, unfortunately, it does not solve my problem.
Maybe my first description was misleading. I meant that Orion is even unable to initially clone the git repository from Gerrit. It just keeps asking for a private key.
Ok, all I can say is I'm not aware of any bugs in this area (which doesn't mean there aren't any). I have repositories cloned in orion using both ssh and git: protocols, and using either passphrase or private key setups without problems. Maybe there is something different about the authentication on that server but it's hard to say without a way to try it out.