users cannot login when one domain controller is down

We have 4 domain controllers, windows 2003 SP1. 2 of them are global
catalogs. Whenever a server is down for maintenance the users are
unable to logon with their roaming profile. They don't have access to
all disks on the DFS even if the server that has been shutted down has
nothing to do with the DFS (in another words, it is not a DFS root and
it does't host any disk)

I checked Sites and services and the even viewer. Nothing wrong is
reported.

Maybe someone has an advice...

Thank you

Sebastien