When you try to password protect a WordPress subdirectory by adding
.htpasswd under subdirectory, you sometimes (in fact most of the time) get Error 404 – Page not found when you visit that subdirectory location in the browser.
To fix this error, add the following line at the top of
.htaccess located in the root of your WordPress installation (yes, root .htaccess and not .htaccess of your password protected subdirectory), and you're good to go now.
ErrorDocument 401 default