Given the error message and the code, it looks like in the line new Value($fields, 'array') in the fetchData method, $fields is an array of plain php strings. It should be an array of XmlRpc\Value objects.

You can either do that conversion manually, or simplify your code by having recursive wrapping of native php values into XmlRpc\Value objects by using XmlRpc\Encoder::encode() method, as seen f.e. in https://github.com/gggeek/phpxmlrpc/blob/master/demo/client/proxy.php