On the server where dbFrontService is installed you can open the Logfile and it will give you more details.
By default you can find the log file at: C:\Logs\dbFrontService\Log_000.log
You should find something like the following...
Error: Can't validate user, Access Violation: User [%username%] is not
a member of [%usergroup%]
This means that the users name and password were correct but that they were not a member of the required user group. This likely means that someone made network changes at your location.