Random login issues

Been using Watu Pro for several months and it has worked fine. Now, suddenly just random persons can login. The get the message “You are not in the allowed user account for this exam. You are not allowed to access this exam at the moment”.
We have not changed how we give them access to the exam. They are registered users and we put their email addresses in the User and Email Related Settings box like we have always done. Sometimes if I go into the user’s account and just try changing the password and just re-save it, it might start to work. We also have users who have no problem logging in. There is no pattern, just random issues.

The page I need help with: [log in to see the link]