Hm, perhaps this get's really freenas related - so probably more help in the forum there?
From my experience, when I select a domain user, there is a kinit process starting message in freenas. So is kerberos running?
For further help some screenshots would be perhaps helpful. I don't know much about the system your running. Is it the new beta-gui? Did you already add users to freenas? etc. Multiple setups can still give same errors for different reasons...
And yes, sometimes a fresh start makes sense - in my opinion.