Got them to do that, to clear their temporary files cache etc.
It just goes straight to the login error page without first prompting for a log in box.
This is why I'm thinking they had entered an incorrect password and IE is autocompleting the form for them. But they insist they cleared the stored passwords.
I even set up a new protected area called test and created totally new, fresh login details so that it couldnt be cached or stored in autocomplete form. Again, they dont get an error but just go straight to the error login page.
Bit of a weird one this. I hate it when a problem affects some people and not others! I'd much rather it to be totally working or totally broken.