Trouble Logging In, Please Help

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We are using PHRETS API to log in to 'http://retsgw.flexmls.com/rets2_2/Login' under username 'met.rets.olo.remx', RETS version 1.5, and we are getting this error:

Fatal error: Uncaught exception 'Exception' with message 'String could not be parsed as XML' in /var/www/html/PHRETS/vendor/guzzlehttp/guzzle/src/Message/Response.php:168 Stack trace: #0 /var/www/html/PHRETS/vendor/guzzlehttp/guzzle/src/Message/Response.php(168): SimpleXMLElement->__construct('xml() #2 /var/www/html/PHRETS/remax_realty_100/get_csv.php(16): PHRETS\Session->Login() #3 {main} Next exception 'GuzzleHttp\Exception\XmlParseException' with message 'Unable to parse response body into XML: String could not be parsed as XML' in /var/www/html/PHRETS/vendor/guzzlehttp/guzzle/src/Message/Response.php:174 Stack trace: #0 /var/www/html/PHRETS/vendor/troydavisson/phrets/src/Session.php(96): GuzzleHttp\Message\Response->xml() #1 /var/www/html/PHRETS/remax_realty_100/get_csv.php(16): PHRETS\Session->Login() #2 {main} thrown in /var/www/html/PHRETS/vendor/guzzlehttp/guzzle/src/Message/Response.php on line 174