Access Error with IIS6 (Server found request content type ..

Hallo,

I have a PDA (WinCE 4.2) application which using a XML-Webservice for data
exchanging.
Both parts are developed with VS2003. When I'm using IIS5.1 the system runs
without problems.
But if I use an Win2003 Server with IIS6 for my webservice I am receiving
the error:
Server found request content type to be 'text/html', but expected 'text/xml'

In the logfile I have found:
.... 80 HTTP/1.1 POST /xxx/yyy.asmx%00 400 - URL
.... 80 HTTP/0.0 Invalid - 400 - Verb

I can not understand why the the PDA application works with IIS5.1 and not
with IIS6.0.
Any Help?

Thanks
Siegfried