If a profile is invalid, the user will get an error upon logging in.
To locate invalid profiles, go to the manage users section of your admin panel. At the top near the search boxes, you will see an icon to move all invlid profiles to the top of the page. Once selcted all invlid profiles will bubble to the top.
From there, any user that has issues-most often the user does not have a profile set up properly- will have a red exclamation point.
To fix the error, simply click on the blue profile expansion icon.
This will open the user information. The issue will be highlighted with another red exclamation mark.
Make the changes to correct the profile, and click save.