I had set up mirroring for one of my repos to push to a repo on GitHub, and it was working fine until last week when it suddenly stopped working.
Here is the error that I am getting:
13:close stream to gitaly-ruby: rpc error: code = Unknown
desc = Gitlab::Git::RepositoryMirroring::RemoteError: Host key verification failed.
fatal: Could not read from remote repository.
Please make sure you have the correct access rights and the repository exists.
this sounds like a problem with SSH host key verification, please subscribe this issue which sounds like your problem. Maybe the suggested workaround with https works for you.