My friend and I were playing on his server using the snapshot specified in the report. He rode a horse and got kicked. Despite he was riding the horse, when he logged off, the horse didn't despawn. Whenever my friend attempted to log in, the server kicked him with reason "Internal error: java.NullPointerException" or something similar. Killing the horse didn't fix it: when my friend attempted to log in again, he got kicked with the same reason, and the horse spawned again.