@jtappen From what you wrote it seems like you are on the right track with this - Active Directory Defaults in the FOG settings should be the was to go.

So which way do you actually register your clients?

Our Fog is on versoin 1.5.6

This is not the latest version. I cannot say for sure we have found und fixed in the latest but if we find this to be a bug I need you to update to the very latest version.