[19:37:14 ERROR]: Encountered an unexpected exception
java.util.ConcurrentModificationException
        at java.util.LinkedList$ListItr.checkForComodification(Unknown Source) ~[?:1.8.0_144]
        at java.util.LinkedList$ListItr.next(Unknown Source) ~[?:1.8.0_144]
        at net.minecraft.server.v1_9_R1.StructureStart.a(SourceFile:40) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at net.minecraft.server.v1_9_R1.StructureGenerator.a(SourceFile:94) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at net.minecraft.server.v1_9_R1.ChunkProviderGenerate.recreateStructures(SourceFile:430) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at org.bukkit.craftbukkit.v1_9_R1.generator.NormalChunkGenerator.recreateStructures(NormalChunkGenerator.java:41) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at net.minecraft.server.v1_9_R1.Chunk.a(Chunk.java:871) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at net.minecraft.server.v1_9_R1.Chunk.loadNearby(Chunk.java:843) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at org.bukkit.craftbukkit.v1_9_R1.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:71) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at org.bukkit.craftbukkit.v1_9_R1.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:1) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at org.bukkit.craftbukkit.v1_9_R1.util.AsynchronousExecutor$Task.finish(AsynchronousExecutor.java:188) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at org.bukkit.craftbukkit.v1_9_R1.util.AsynchronousExecutor.finishActive(AsynchronousExecutor.java:347) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at org.bukkit.craftbukkit.v1_9_R1.chunkio.ChunkIOExecutor.tick(ChunkIOExecutor.java:34) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at net.minecraft.server.v1_9_R1.MinecraftServer.D(MinecraftServer.java:693) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at net.minecraft.server.v1_9_R1.DedicatedServer.D(DedicatedServer.java:361) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at net.minecraft.server.v1_9_R1.MinecraftServer.C(MinecraftServer.java:635) ~[craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at net.minecraft.server.v1_9_R1.MinecraftServer.run(MinecraftServer.java:539) [craftbukkit-1.9.jar:git-Bukkit-ab83272]
        at java.lang.Thread.run(Unknown Source) [?:1.8.0_144]