ISA server 2006 all uses are getting password prompt.

Dear Team,

We having ISA server 2006 configure for proxy, randomly all uses are getting password prompt while accessing any website, password not accepting.

1. We have configured websense on same server.
2. During proxy issue, As a domain admin I am able to access website from ISA server and from any system of network.
3. No Event is generating in event logs.

OS : Windows server 2003.

As of now we are restarting server and resolving problem.

If anyone have faced this issue pls guide me how to kick this proxy issue.

Regards, Sushant