After I upgraded from IPB 3, my directory protection (require separate password and username) no longer works. I attempted to change the password and username and still doesn't work.

Quote

The page you requested does not exist: 1S160/2

is the error output I get.

It sounds like you have an .htaccess rule in your admin directory which is throwing an error, so the request is being routed to the front end index.php file ultimately.