Deceiving the herd quest bug, can't plant letter

Just as the title says. I'm doing the "deceiving the herd" quest. Every time I kill someone and try to plant the item it says I can't remove a quest item. No I'm not killing indoors, no I'm not using the ebony blade. I am not doing anything to cause this to fail. I have the latest unofficial skyrim patch installed too.


Please tell me there is a fix for this. I can't imagine there isn't. I tried using a cheat to mark the quest complete, but the person who gave me it doesn't recognize it as complete.