Why Authentication Screen?

[Guest]

I have several users who are logging into windows, then click Outlook, which
connects properly. Later in the day, randomley, an Outlook authentication
screen will appear asking the user to login with their ID and password. This
is not normal behavior and I'm trying to determine what network or client
forces could cause this type of response. Event log checks and network
connectivity appear normal.

 

[Guest]

I am having this exact same issue. I cannot determine the cause. any help
would be greatly appreciated.

In addition to the details below I would add that when the screen appears
the users try to enter thier credentials and valid usernames and passwords
are not accepted. This typically will go away after a reboot or logoff, but
not always.