Sending a File via a secure connection with Indy to a website - delphi

Question

I have to send an XML-File to a Website with a secure Connection. Delphi 2010, Indy 10.5.9. The Code used is as follows:
Params: TIdMultiPartFormDataStream;
ResponseStr: string;
begin
result := 0;
sRootCertFile := 'xxx\Digital.pem';
sCertFile := 'xxx\Digital.pem';
sKeyFile := 'xxx\Digital.pem';
with FAdminSetup.RDWSSLHandler do
begin
SSLOptions.VerifyMode := [];
SSLOptions.VerifyDepth := 0;
SSLOptions.RootCertFile := sRootCertFile;
SSLOptions.CertFile := sCertFile;
SSLOptions.KeyFile := sKeyFile;
end;
sURL := 'https://xxx/xxxservice';
begin
IdHttpVVO := TIdHttp.Create(nil);
try
// IdHttpVVO.Request.ContentType := 'multipart/form-data';
// IdHttpVVO.ProtocolVersion := pv1_1;
// IdHttpVVO.HTTPOptions := [hoKeepOrigProtocol,hoForceEncodeParams];
// IdHttpVVO.Request.Connection := 'Keep-Alive';
// IdHttpVVO.Request.CacheControl := 'no-cache';
// IdHttpVVO.Request.ContentLength := Length(sAnsiXML); // <-- new
IdHttpVVO.IOHandler := FAdminSetup.RDWSSLHandler;
Params := TIdMultiPartFormDataStream.Create;
try
with Params do
begin
AddFile('file', filename, GetMIMETypeFromFile(filename));
end;
resultStr := IdHttpVVO.Post(sURL, Params);
finally
Params.Free;
end;
ShowMessage(resultstr);
The result is always the same:
'HTTP/1.0 500 Error'
when doing the post part.
All the remarks have been tried and did not give any change. The Password for the connection is supplied as follows:
procedure TFAdminSetup.RDWSSLHandlerGetPasswordEx(ASender: TObject;
var VPassword: AnsiString; const AIsWrite: Boolean);
begin
VPassword := 'xxx';
end;
The Website is working, the certificates seem to be ok, as there is a small tool included written in C that works.
Where is my mistake? Thanks