Please follow instruction
at this page delete all the files, once you delete you will find 404 error.
Did you delete you cache and browsing history for the browser currently using?
Once you done the above two steps. log out of coursera, then re login. then open assignment page at this time, you will probably see the 404 error.
Next click help=== reboot,
Once rebooting is done, click help again and click the Get the latest version, then Update lab.
Voila you will have a fresh updated copy. Share the image of grader cell Triple loss here to confirm it matches with the image I posted.
Regards
DP