Server was unable to process request. --> Security error

[Newbie]

Hello, I have an ASP.NET 1.1 web service running on IIS 6.0 that I am trying
to access from another machine thru anonymous access. Both machines run under
the same domain and I can access the web service using Internet Explorer from
the client machine.

However, when I try to consume the web service I get the following exception:

"Unhandled Exception: System.Web.Services.Protocols.SoapException: Server
was unable to process request. --> Security error."

I noticed in IIS logs, HTTP GET requests work ok but a POST gives HTTP 500
error...

Any idea what could be missing?

Thanks in advance...