The specific error is:
This error seems to happen only when your computer is behind a firewall or is using IP forwarding for its network address. The kerberos ticket has the true address of your computer in it, and SAP is looking at the apparent address of your computer (often this is the firewall or ip-forwarding-server) and they don't match. When they don't match, SAP thinks you're being tricky and won't let you in.
As of August 2002, you can solve this problem on Windows computers by obtaining "address-less tickets". These tickets apparently do not embed the IP address and pass the validity test in services that use Kerberos, such as SAP and the MIT email servers.
The easiest way to get address-less tickets is to install and use KfW.
To read about alternative methods of obtaining address-less tickets, see Obtaining Address-less Tickets in Kerberos for Windows.