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My guess is that you're somehow modifying tasks or targetTasks from within the AI task itself (e.g. by calling setCombatTask()). This is a no-no as you modify a list while that list is being iterrated over, invalidating the iteration (was the item removed already iterated over? if so, do we need to somehow "undo" it? was the item added supposed to be iterated over? The JVM can't answer these questions).

What happens when water washes away grass and flowers? A Harvest event is fired where event.getHarvester() is null.

This is caused by you either screwing up the install, or not running the server correctly. The issue is that java can not find the vanilla server jar file. It needs to be in the same directory as you run the server, named exactly how vanilla names the jar file. Which is why, that is exactly what the installer does. There isn't much we can do about it but ask you for more information. If you followed instructions on installing, and ran the server normally it will work. Calling our staff assholes is just a way to get banned. So.. Enjoy the ban.