EWS managed api autodiscover error

Can anyone tell me what caused this error?

Thanks in advance

Microsoft.Exchange.WebServices.Data.ServiceXmlDeserializationException: Expected XML node type was XmlDeclaration, but actual type is Element.
   at Microsoft.Exchange.WebServices.Data.EwsXmlReader.Read(XmlNodeType nodeType)
   at Microsoft.Exchange.WebServices.Autodiscover.AutodiscoverService.GetConfigurationSettingsAtUrl[TSettings](String emailAddress, String url)
   at Microsoft.Exchange.WebServices.Autodiscover.AutodiscoverService.InternalGetConfigurationSettings[TSettings](String emailAddress, Int32& currentHop)
   at Microsoft.Exchange.WebServices.Autodiscover.AutodiscoverService.GetConfigurationSettings[TSettings](String emailAddress)
   at Microsoft.Exchange.WebServices.Autodiscover.AutodiscoverService.GetLegacyAutodiscoverEwsUrl(String emailAddress)
   at Microsoft.Exchange.WebServices.Data.ExchangeService.AutodiscoverUrl(String emailAddress, AutodiscoverRedirectionUrlValidationCallback validateRedirectionUrlCallback)