Steps to reproduce :
log in GardenAdmin and goto the Sales Order window ; add some default values.
Now, log in System and export the Sales Order Window.
(Note that the export will fail because some preference don't have UU)
If you have a look at the PackOut.xml, you will see some reference to AD_Preference
You can also have a look at the attached test.zip package
I had the same problem today and in my opinion, there is a simple check missing from which client the preference comes from. So to solve this problem, i created a little patch. Please check if it is ok and if yes, please integrate it into the trunk.
What bothers me more than the fact that a clients preference is packed out when i make a pack out on System, is the fact that the uuid of those preferences is missing.