Bukkit

What steps will reproduce the problem?
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What is the expected output? What do you see instead?

What version of the product are you using?

Do you have an error log of what happened?
8:37:30 AM org.bukkit.event.EventException
8:37:30 AM at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:305) ~[spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM at net.minecraft.server.v1_8_R2.BlockLeaves.e(BlockLeaves.java:137) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM at net.minecraft.server.v1_8_R2.BlockLeaves.b(BlockLeaves.java:127) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM at net.minecraft.server.v1_8_R2.Block.a(Block.java:299) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM at net.minecraft.server.v1_8_R2.WorldServer.h(WorldServer.java:494) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM at net.minecraft.server.v1_8_R2.WorldServer.doTick(WorldServer.java:252) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM at net.minecraft.server.v1_8_R2.MinecraftServer.A(MinecraftServer.java:765) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM at net.minecraft.server.v1_8_R2.DedicatedServer.A(DedicatedServer.java:368) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM at net.minecraft.server.v1_8_R2.MinecraftServer.z(MinecraftServer.java:651) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM at net.minecraft.server.v1_8_R2.MinecraftServer.run(MinecraftServer.java:554) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM at java.lang.Thread.run(Unknown Source) [?:1.8.0_40]
8:37:30 AM Caused by: java.lang.IncompatibleClassChangeError: Found interface com.sk89q.worldguard.protection.ApplicableRegionSet, but class was expected
8:37:30 AM at de.TrustedCreeper.NaturalTrees.NaturalTreesEventHandler.onLeavesDecay(NaturalTreesEventHandler.java:38) ~[?:?]
8:37:30 AM at sun.reflect.GeneratedMethodAccessor256.invoke(Unknown Source) ~[?:?]
8:37:30 AM at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_40]
8:37:30 AM at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_40]
8:37:30 AM at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:301) ~[spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:30 AM ... 13 more
8:37:31 AM [ERROR] Could not pass event LeavesDecayEvent to NaturalTrees v1.1.0
8:37:31 AM org.bukkit.event.EventException
8:37:31 AM at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:305) ~[spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:31 AM at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:31 AM at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:31 AM at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:31 AM at net.minecraft.server.v1_8_R2.BlockLeaves.e(BlockLeaves.java:137) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:31 AM at net.minecraft.server.v1_8_R2.BlockLeaves.b(BlockLeaves.java:127) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:31 AM at net.minecraft.server.v1_8_R2.Block.a(Block.java:299) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:31 AM at net.minecraft.server.v1_8_R2.WorldServer.h(WorldServer.java:494) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:31 AM at net.minecraft.server.v1_8_R2.WorldServer.doTick(WorldServer.java:252) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:31 AM at net.minecraft.server.v1_8_R2.MinecraftServer.A(MinecraftServer.java:765) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:31 AM at net.minecraft.server.v1_8_R2.DedicatedServer.A(DedicatedServer.java:368) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:31 AM at net.minecraft.server.v1_8_R2.MinecraftServer.z(MinecraftServer.java:651) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:31 AM at net.minecraft.server.v1_8_R2.MinecraftServer.run(MinecraftServer.java:554) [spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:31 AM at java.lang.Thread.run(Unknown Source) [?:1.8.0_40]
8:37:32 AM Caused by: java.lang.IncompatibleClassChangeError: Found interface com.sk89q.worldguard.protection.ApplicableRegionSet, but class was expected
8:37:32 AM at de.TrustedCreeper.NaturalTrees.NaturalTreesEventHandler.onLeavesDecay(NaturalTreesEventHandler.java:38) ~[?:?]
8:37:32 AM at sun.reflect.GeneratedMethodAccessor256.invoke(Unknown Source) ~[?:?]
8:37:32 AM at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_40]
8:37:32 AM at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_40]
8:37:32 AM at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:301) ~[spigot-1.8.3.jar:git-Spigot-dbe012b-61ef214]
8:37:32 AM ... 13 more

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having same issue, have you found out how to fix it?