jetty: use j_security_check as url returns 500 error

Question

I use jetty FormAuthenticator to validate the login data like this

FormAuthenticator authenticator = new FormAuthenticator("login",  "login", false);

In the login form, I have

<form method='POST' action='app/j_security_check'>
</form>

The problem is when I run the tests, if I type this url

localhost:8080/app/j_security_check

I get an 500 error. All the other incorrect url cases can be re-directed except this one. Is there any way to handle this? I tried with filter and it does not work. Thanks