The most likely problem is that dbFront authentication is configured incorrectly. The domain is either invalid or can't be found.
By default dbFront starts with a period ".". This allows dbFront to use the domain the current server is associated with or if none is specified it will use the current computer.
After installation, you should manually specify the domain name and set the ContextType to "Domain" to ensure that dbFront used the correct authentication methods.
All authentication information is configured on the first tab of the settings screen.
For more details see: http://dbfront.com/windowsauthentication
Another possibility is that the domain is valid but not currently reachable. You might be offline or out of the office.
If that is the case then you either need to reconnect to your domain or use the local machine credentials.