An error I do not understand

For some time now I have been trying to figure out what is causing this bug in the console:

Code:

[05:10:55] [Server thread/ERROR]: Could not pass event EntityDamageByBlockEvent to skyWars v1.8.4
org.bukkit.event.EventException
    at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:294) ~[spigot.jar:git-Spigot-1371]
    at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[spigot.jar:git-Spigot-1371]
    at org.bukkit.plugin.TimedRegisteredListener.callEvent(TimedRegisteredListener.java:30) ~[spigot.jar:git-Spigot-1371]
    at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [spigot.jar:git-Spigot-1371]
    at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [spigot.jar:git-Spigot-1371]
    at org.bukkit.craftbukkit.v1_7_R2.event.CraftEventFactory.callEvent(CraftEventFactory.java:99) [spigot.jar:git-Spigot-1371]
    at org.bukkit.craftbukkit.v1_7_R2.event.CraftEventFactory.handleEntityDamageEvent(CraftEventFactory.java:432) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.EntityLiving.damageEntity(EntityLiving.java:663) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.EntityAnimal.damageEntity(SourceFile:145) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.EntityLiving.F(EntityLiving.java:1020) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.Entity.B(Entity.java:348) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.EntityLiving.B(EntityLiving.java:159) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.EntityInsentient.B(EntityInsentient.java:111) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.Entity.h(Entity.java:255) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.EntityLiving.h(EntityLiving.java:1273) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.EntityInsentient.h(EntityInsentient.java:150) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.World.entityJoinedWorld(World.java:1486) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.World.playerJoinedWorld(World.java:1462) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.World.tickEntities(World.java:1329) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.WorldServer.tickEntities(WorldServer.java:515) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.MinecraftServer.v(MinecraftServer.java:676) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.DedicatedServer.v(DedicatedServer.java:273) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.MinecraftServer.u(MinecraftServer.java:566) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.MinecraftServer.run(MinecraftServer.java:472) [spigot.jar:git-Spigot-1371]
    at net.minecraft.server.v1_7_R2.ThreadServerApplication.run(SourceFile:618) [spigot.jar:git-Spigot-1371

Does anyone know what could cause this?

 

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