It sounds like someone has mucked about with the linux user account called fog. We have seen some people use the linux fog account for system maintenance and other activities. This is an internal account and should be used by the FOG system only.
It sounds like you have started checking the spots for this user account. I would look in this order.
Review the /opt/fog/.fogsettings file for the password. Ensure you can log into the FOG server from a windows ftp program using uid= fog and passwd= <password listed in .fogsettings>. If that works move on to step 3 In the web gui, look at the storage node configuration for the master server. At the bottom there should be a user ID and password. Make sure that password is the same that is found in the .fogsettings file. Look at FOG Configuration->FOG Settings, in the tftp section there is also a user ID and password that needs to be consistent with the .fogsettings file.If all of the test pass then post back here (either way) and we can dig a bit deeper into the error message, but I believe either the linux user fog’s password has been changed or one of the checks above will point to the problem.
I might also suggest (once you get the password issue worked out), that you upgrade to fog 1.4.4, because 1.4.0 had several issues with the single disk resizeable option, that was addressed in 1.4.1 and again in 1.4.4. It is in your best interest to upgrade once the password issue has been fixed.