OK thanks,
I'm still trying to figure this out, too! 
Is thin provisioning being used in this environment? I am thinking the allocation versus capacity may be the factor. In my lab, I am getting the same error that you are seeing, even though my disk space usage trending is well below my capacity. If I select my datastore, and go to the Planning tab under Summary - adjust the Resources table for Unit: Percentage and I see that I am over allocated. Is this your situation? Screen cap from my lab for reference....
