Delphi XE 6 - SOAP request HTTPRIO error: The system cannot find the file specified

Question

The error appears in the ShowMessage(aServer.mc_version) line of this code:

uses mantisconnect;

procedure TForm.Button1Click(Sender: TObject);
var
   aServer: MantisConnectPortType;
begin
   aServer := GetMantisConnectPortType;
   ShowMessage('Mantis version:' + aServer.mc_version);
end;

The same code works on Delphi XE5, but compiled on Delphi XE6 triggers an error the first time (first click on the button), but it works the next time:

The system cannot find the file specified 
URL:http://www.mymantis.com/api/soap/mantisconnect.php - 
SOAPAction: URL:http://www.mymantis.com/api/soap/mantisconnect.php/mc_version

If I try again (second click on the button) it works! Output:

Mantis version:1.2.9

Listing of the connection procedure is

function GetMantisConnectPortType(UseWSDL: Boolean; Addr: string; HTTPRIO: THTTPRIO): MantisConnectPortType;
const
  defWSDL = 'http://www.mymantis.com/api/soap/mantisconnect.php?wsdl';
  defURL  = 'http://www.mymantis.com/api/soap/mantisconnect.php';
  defSvc  = 'MantisConnect';
  defPrt  = 'MantisConnectPort';
var
  RIO: THTTPRIO;
begin
  Result := nil;
  if (Addr = '') then
  begin
    if UseWSDL then
      Addr := defWSDL
    else
      Addr := defURL;
  end;
  if HTTPRIO = nil then
    RIO := THTTPRIO.Create(nil)
  else
    RIO := HTTPRIO;
  try
    Result := (RIO as MantisConnectPortType);
    if UseWSDL then
    begin
      RIO.WSDLLocation := Addr;
      RIO.Service := defSvc;
      RIO.Port := defPrt;
    end else
      RIO.URL := Addr;
  finally
    if (Result = nil) and (HTTPRIO = nil) then
      RIO.Free;
  end;
end;

mc_version is a part of

MantisConnectPortType = interface(IInvokable)

and has the declaration:

function  mc_version: string; stdcall;