Unable to create envelop : class com.sun.xml.messaging.saaj.soap.dom4j.ElementImpl cannot be cast to class com.sun.xml.messaging.saaj.soap.Envelope

I'm trying to POST SOAP request to sabre in c# console application but it gives an error. I have checked error details it says i may be having some special characters before soap body or in soap body but i didn't have any.

Error Details -

<?xml version="1.0" encoding="UTF-8"?>
<soap-env:Envelope xmlns:soap-env="http://schemas.xmlsoap.org/soap/envelope/"><soap-env:Header/><soap-env:Body><soap-env:Fault><faultcode>soap-env:Client.InvalidEbXmlMessage</faultcode><faultstring>Unable to create envelope from given source: class com.sun.xml.messaging.saaj.soap.dom4j.ElementImpl cannot be cast to class com.sun.xml.messaging.saaj.soap.Envelope (com.sun.xml.messaging.saaj.soap.dom4j.ElementImpl and com.sun.xml.messaging.saaj.soap.Envelope are in unnamed module of loader org.springframework.boot.loader.launch.LaunchedClassLoader @6950e31)</faultstring><detail><StackTrace>javax.xml.soap.SOAPException: Unable to create envelope from given source: class com.sun.xml.messaging.saaj.soap.dom4j.ElementImpl cannot be cast to class com.sun.xml.messaging.saaj.soap.Envelope (com.sun.xml.messaging.saaj.soap.dom4j.ElementImpl and com.sun.xml.messaging.saaj.soap.Envelope are in unnamed module of loader org.springframework.boot.loader.launch.LaunchedClassLoader @6950e31)</StackTrace></detail></soap-env:Fault></soap-env:Body></soap-env:Envelope>

C# SOAP Request Code -

var client3 = new HttpClient();
var request3 = new HttpRequestMessage(HttpMethod.Post, "https://webservices.platform.sabre.com");
request3.Headers.Add("Authorization", d);
request3.Headers.Add("Cookie", "visid_incap_2768617=kSlKoHeWSX6k4I6cGPIi5sCMk2cAAAAAQUIPAAAAAAAQ/smeEN9Nwzlur1q5LTyI");
var content3 = new StringContent("<?xml version=\"1.0\" encoding=\"UTF-8\" ?><SabreCommandLLSRQ ReturnHostCommand=\"true\" Version=\"2.0.0\" xmlns=\"http://webservices.sabre.com/sabreXML/2011/10\"><Request Output=\"SCREEN\"><HostCommand>WETRT*6952836690549</HostCommand></Request></SabreCommandLLSRQ>", Encoding.UTF8, "text/xml");
request3.Content = content3;
var response3 = client3.SendAsync(request3).Result;
//response.EnsureSuccessStatusCode();
var b = await response3.Content.ReadAsStringAsync();

Note: InPostman This SOAP Request is working for me but not in c# console application.