If you get a 500 error back, look at the exact contents of the error message () the problem

    $response = $ua->request($request);
    if ($response->is_success) {
        print $response->decoded_content;
    }
    else {
        print STDERR $response->status_line, "\n";
    }
[download]

Maybe there is some error on your side, maybe there is some error with the remote server, but without the error message, it's hard to tell where the error is.