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

Start from the Repository.Network.Push call and its CredentialsProvider in the report, comparing UsernamePasswordCredentials with SshUserKeyCredentials. Reproduce the push against the GitLab remote and investigate the reported “unable to exchange encryption keys” failure; done means the SSH session starts and the push succeeds with the supplied credentials.