Fix A Database Issue With Account Creation/Login Problems (Java / MangoDB)

Our Java based web database (MangoDB) is experiencing issues with user accounts including problems logging in with some accounts (returning "auth-failed" message with correct password) and problems creating accounts. We need you to find the problem and implement a permanent fix to prevent this from happening in the future. Note: This issue has occurred a few times in the past, and our previous developer was able to fix it within around 15 minutes (unfortunately he is unavailable for this project), so it should be a simple project.