SharpSvn giving SASL(-1) error when used on a different server

Question

I am trying to use the SharpSvn library on our build server. On the development machine it works fine but when I put on the build server I get the following error:

SASL(-1): generic failure: Unable to find a callback: 2

The code I am using is as follows:

    public IList<Branch> GetBranches()
    {
        using (var client = new SvnClient())
        {
            Collection<SvnListEventArgs> outBranches;
            client.Authentication.Clear();
            client.Authentication.UserNamePasswordHandlers += Authentication_UserNamePasswordHandlers;
            client.GetList(SvnTarget.FromString(_settings.SvnBranchUrl), out outBranches);
            return outBranches.Select(x => new Branch {Name = x.Path}).ToList();
        }
    }

    private void Authentication_UserNamePasswordHandlers(object sender, SharpSvn.Security.SvnUserNamePasswordEventArgs e)
    {
        e.UserName = _settings.SvnUsername;
        e.Password = _settings.SvnPassword;
    }

TIA

Andrew