If you are hostile to anyone, no matter where they are, you cannot claim this land.
I am unable to claim this farm anymore myself. Meta-gaming a bit I checked and had no hostile PC's on the list and walked entire map (in plain sight) two or three times and no stealth or hostile creatures showed up. Is it a bug or is there a set amount of times you can claim this farm?
I'm not sure if it's bugged, but I was able to claim the farm yesterday just fine.