Authentication dying

Hello.

I have a simple discussion forum app which is redirected to by a standard login app.

The link to the forum is

http://server/path/db.nsf/Viewname?OpenView?login&username=UID&password=PWD &RedirectTo=/path/db.nsf/Viewname?OpenView

The link works, sometimes.

The UID & PWD are correct.

Sometimes, though, I get a screen asking for authentication saying that my login has expired (many of my users get this screen). However, I’m passing the UID and PWD - why is it complaining?