Please explain: Your credentials have expired

[Petrus de Calguarium]

I just updated my f15 system to yesterday's updates (08.iii.2011), those 
including kernel-2.6.38-0.rc8.git0.1.fc15 &c.

I rebooted the machine, as I wanted to run the new kernel.

When kdm logged me in, I saw 2 keys with a red 'x' in the system tray. Hovering 
over it shows "Your credentials have expired" and when I clicked it, an empty 
window appeared, and after a long, long delay, it filled in with the following 
information:

Couldn't acquire kerberos ticket: 'Cannot resolve network address for KDC in 
realm "EXAMPLE.COM"'

What is this and why is it appearing? I don't know what kerberos is and I don't 
think I have used it in 12 years of linuxing.

What do I do about it to fix it?