The answer comes from the proxy, not from the website: your request never got that far. The usual causes are a stray space copied with the password, a username that the program shortened, a password that belongs to a different plan, or a program that does not send proxy logins at all.
Copy the username and password again from your plan page and try the simplest possible test, such as the curl command in the docs. If that works, the login is fine and the problem is in how the other program passes it. A plan that has run out of traffic or time can also answer this way.
Read next: Troubleshooting: 407, timeouts and wrong countries