Internal Server Error (500) in Upload File method

[P.Belo]

I am using a WebClient.UploadFile method (using the HTTP POST) on the
Client side and, to accept posted files on the Server side, I am using
an ASP.NET page with the following code:

void Page_Load(object sender, EventArgs e)
{
foreach(string f in Request.Files.AllKeys)
{
HttpPostedFile file = Request.Files[f];
file.SaveAs("c:\\WUTemp\\" + file.FileName);
}
}

When Client and Server side are on the same machine(localhost/127.0.0.1)
everything runs ok but when I use the same ASP.NET page in a different
server and call it from the WebClient.UploadFile method
(WebClient.UploadFile("http://srv1/ASPSavePostFile/Upld.aspx", "POST",
fileToUpload)), it returns the error: "(500) Internal Server Error".
I've already tried to solve this problem with all the instructions I
found in Microsoft and MSDN but with no results. I have no idea what to
try now and I would appreciate if someone could help me to follow in the
right direction.
Thanks for your help.
P.Belo