Delete the following two files from your administrator folder: .htaccess and .htpasswd. Please note that the file names start with a dot which makes them hidden. You will need to show hidden files in your FTP/SFTP programme or your hosting control panel's file manager to see and remove them. This will turn off the administrator password protection.
To understand why this happens and why you'll have to talk to your host about resolving this issue please read https://www.akeeba.com/documentation/admin-tools/atadminpw.html under "404 Not Found error page or Joomla error page when enabling this feature". This applies to your case as well. Your server's custom error pages for HTTP 401 seem to be served with the wrong HTTP status (200 OK instead of 401 Authorisation Required). Since password protection works at the server level with the server sending a 401 status to your browser to force it to ask you for a username and password, if the server messes up the status response this feature will not work. Also note that this feature is actually not implemented in Admin Tools itself; we simply create the .htaccess and .htpasswd files to let your server handle password authentication for the entire administrator folder.
Nicholas K. Dionysopoulos
Lead Developer and Director
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