We have a windows service running that listens on e.g. port 9999. Now we want to be able to send a TCP packet through an ASPX page to that service. Both the service and ASP.NET WebApp run on the same server.

The problem is that this works fine on my dev machine (without IIS though, through Visual Studio Web Server) and on our older Win 2003 Server machine. But it doesn't work on our new Win2008 R2 Server. The packet just won't arrive.

I'm absolutely clueless as to what the problem may be. A search in Google didn't help me as it seems I'm the only one wanting to send TCP packets through an aspx site.

What I have tried so far:
1) Changing the identity of the corresponding Application Pool
2) Disabled the Firewall
3) In web.config set the trust level to "Full" (which seems to be default anyways)

I'd appreciate any more hints!