Hi Ria,
Though you are partially correct, the reason why we get 404 error at times on openiing MII workbench files is a little different.
Whenever a user launches or runs a file from workbench, the default browser configured for that particular system gets initialised. Everytime when you launch any workbench file, it will open up in your system's default browser. If there is no other active MII session going on, then a logon screen appears and on giving the credentials, the launched page opens up without any problems.
As opposed to this, if you have already logged into MII using say IE, but the system's default browser is Google Chrome, then on launching a file from workbench, a Chrome window will open, you still WILL see the logon screen, but this time, when you provide the credentials, it will lead you to 404 error.
Hence, it is a good practise to open MII session in the same browser which is your system's default browser.
Regards,
Rutika Bodas