1- You have entered an incorrect URL where the URL format or the Database user ID is incorrect. For instance, you have entered
http://<server>:<port>/ws-legacy/debug.jsp?secret=<database User ID>
The URL used to access debug.jsp should be in the following format.
http://<server>:<port>/ws/debug.jsp?secret=database User ID
and you should make sure that you use the database user ID (without the "<>") and not the database user ID password.
Moreover: the Database User ID is case-sensitive. You can check to make sure you have the right database user id by viewing the startup.log file in Administrations-->System Logs
2- The debug_jsp URL is correct, but you are using WorldServer version 11.8.x and you are not logged in to WorldServer as an Administrator. You need to log in to WorldServer as a user with User Type Administrator with your username and password before entering the debug_jsp URL in your browser. This requirement has been introduced starting from Version 11.8.0. It was not necessary in earlier versions.
Once logged in, enter the debug_jsp URL in your browser, and the page will load correctly.