Vista - Intermittent login failure

Login very often fails in the hospital network where I am supporting WLM.
Sometimes it works as it should, but frequently an hour later it fails for
half a day. This is not due to MSN servers, since at the same time,
outpatient users in the same city still can log in normally.

The WLM log file accompanying the log-in procedure gives no specific hint.
It only shows that the first log in attempt by TCP/IP is "hanging" so that
a second and third attempt by socks and by port 80 is not begun. What
factors within our network/firewall might cause this behavior?

Many thanks for every hint, Thomas