failed to start SSH session: unable to exchange encryption keys

        var r = new Repository(working);
        r.Network.Remotes.Add("remote", @"git@gitlab.com:user1234/test.git");

        r.Network.Push(r.Network.Remotes["remote"], @"refs/heads/master", new PushOptions
        {
            CredentialsProvider =
            //(url, user, credentials) => new UsernamePasswordCredentials
            //{
            //    Username = "user1234",
            //    Password = "********"
            //}
            (url, user, credentials) => new SshUserKeyCredentials
            {
                Username = "git",
                Passphrase = string.Empty,
                PublicKey = @"C:\Users\me\.ssh\gitlab.pub",
                PrivateKey = @"C:\Users\me\.ssh\gitlab"
            }
        });

both UsernamePasswordCredentials and SshUserKeyCredentials (openssh keys already added to gitlab) give:

failed to start SSH session: unable to exchange encryption keys

レポート内の Repository.Network.Push 呼び出しとその CredentialsProvider から始め、UsernamePasswordCredentials と SshUserKeyCredentials を比較します。GitLab の remote に対する push を再現し、報告された「unable to exchange encryption keys」エラーを調査します。SSH セッションが開始され、提供された認証情報で push が成功すれば完了です。