I am having a similar problem. The arcgis soc account has the needed permissions but when I try to register a UNC folder i usually receive a "Invalid folder location. Unable to access this location." message.
Oddly enough SOMETIMES it works and will validate. Then a short while later it fails to validate! Very strange.
Interestingly I can also use the exact same folders as used for the server directories, and those fail when used as Data Store directories. They work perfectly fine when configured as server directories! The server all runs under the same ArcSOC account so this makes no sense.
Did you find a resolution?
Eric