Are users using the same credentials as the current one when logging to QlikView? Note that QlikView allows to pass on to the Server the current user credentials in some zones.
It's worth checking your IE settings, if possible, in Control Panel, Internet Options, Security, Local Intranet (or the zone where your QlikView Server is), Custom Level... button, User Authentication, "Automatic Logon only if Intranet zone" or "Automatic Logon with current username and password".
As opposed to Firefox and Chrome, IE is integrated into Windows, so if it's set to only identify when in the Intranet, then the site may ask for credentials.
Does that happen in a new fresh installation of QlikView or this version has been an upgrade from a previous version? Does it always forward you to the login2.htm ? Did you try using the alternate login page for IE?