Might this help you?
Taken from it:
"I have noticed that after updating the system and
periodically now and again the mapped drives on the
server show with the red x. Like they are not
available, but then when I double click on them and
the folders come right up, and then the red x goes
away. It is very confusing. This has bee happening to
many of the clients as well."
"The red-X symptom typically occurs because Windows 2000 Server drops idle
connections after a specified time-out period, which by default is 15
minutes (Autodisconnect), so that server resources are not wasted on unused
sessions. The connection can be re-established very quickly at a later time,
if required and hence the red-X goes away once you access the server. I
don't think that this should cause any issues in your environment."
I hope this helps