Very sorry for not getting back to you sooner.
When you enter the wrong username/password, you really shouldn't be getting to a 404 page. A 404 error is for a page that doesn't exist, and if it's coming up after a bad login, then something sounds wrong with your configuration. For instance, if you try to login on our demo site, you'll simply be left on the same page.. not redirected to a 404 page (that site is J1.5, but 1.7 behaves the same):
www.sourcecoast.com/jfbconnect/demo/demo-1-registerlogin
If that's not the case, and you have something configured to redirect the user to a special "Username/password" incorrect page, let us know how that is configured so we understand more.
Finally, can you let me know if you have the "Returning User Redirection" enabled? If that's set to no, then JFBConnect will leave the user on the same page they were on when they tried to log in. If that happens to be a 'bad' page, there's no way for JFBConnect to be able to detect that, so that's possible. Again, we'd need to understand more about how the redirection to the initial 404 HTML error page is occurring in the first place to help understand how to fix (and that's likely what would need to be fixed, not JFBConnect).
Hope that helps explain, and sorry again for the delay,
Alex