When logging in from 'https://<instance_name>.service-now.com/side_door.do', The page you are looking for could not be found is appearing.
A logged in user cannot access the side_door.do page. If a user attempts to access the page while logged in, it will produce 'page not found' error
When a user doesn't log out of the instance and just close the browser. Logout from the instance properly to access the side_door.do page.
Here is the doc about it: side_door.do page not found