'text/html' expected 'text/xml' WebService error

[Saya]

Hello,

The situation is as follows:
My server app (ASP.NET WebService) responds to the client request
(ASP.NET) by returning a text string appended with 2 int numbers,
being the X&Y coordinate values of a MouseDown event on the client
form. These numbers are supplied by the client.
Code snippet:
--------------
Client-app: myLabel.Text=srvControl.MouseCoordinates(mouseEv.X,
mouseEv.Y);

Server-app:
[Web Method]
public string MouseCoordinates(int nX,int nY)
{
return "Your Mouse Coordinates: "+nx.ToString()+","+nY.ToString();
}
---------------
When I run this code, on a wireless SmartDevice TabletPC, a weird
thing happens: if I click my mouse down at a 'slow' pace, everything
runs OK. If I do it fast, the app crashed with the following message:
<< Error
myWebApp.exe InvalidOperationException
Server found request content to be 'text/html' but expected
'text/xml'.

Application::Run+0xf >>

Looking into the W3C-logfile: error 403 (Forbidden)
The MS-IIS logfile has more: 403.5 (SSL 128 required)

I can't believe these error messages are true for this very-very
simple request.
I tend to look into some timing parameter in the Server area, because
if I ran this code on the SmartDevice emulator, it did not break.

Any ideas please ... I'm desperate ... .

Thank you,Saya