Bukkit

I am not using ProtocolLib ability because I want the following features that it lacks:

• Snow to be placed on surfaces other than grass.
• Ice to spread out from the shore.
• Snow to replace tallgrass (in conjunction with SnowControl).
• Snow to fall through certain blocks (in conjunction with SnowControl).

So, this did work all fine without ProtocolLib.  However, my issue arises when attempting to change seasons.  I receive "Exception ticking world" from "java.util.ConcurrentModificationException".  Thus, the server crashes.  Full log: http://pastebin.com/DetKryUJ

[23:22:05] [Server thread/ERROR]: Encountered an unexpected exception
net.minecraft.server.v1_11_R1.ReportedException: Exception ticking world
	at net.minecraft.server.v1_11_R1.MinecraftServer.D(MinecraftServer.java:801) ~[spigot-1.11.2.jar:git-Spigot-7d78b81-27dd3ca]
	at net.minecraft.server.v1_11_R1.DedicatedServer.D(DedicatedServer.java:399) ~[spigot-1.11.2.jar:git-Spigot-7d78b81-27dd3ca]
	at net.minecraft.server.v1_11_R1.MinecraftServer.C(MinecraftServer.java:675) ~[spigot-1.11.2.jar:git-Spigot-7d78b81-27dd3ca]
	at net.minecraft.server.v1_11_R1.MinecraftServer.run(MinecraftServer.java:574) [spigot-1.11.2.jar:git-Spigot-7d78b81-27dd3ca]
	at java.lang.Thread.run(Thread.java:745) [?:1.8.0_40]
Caused by: java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429) ~[?:1.8.0_40]
	at java.util.HashMap$KeyIterator.next(HashMap.java:1453) ~[?:1.8.0_40]
	at net.minecraft.server.v1_11_R1.PlayerChunkMap.flush(PlayerChunkMap.java:122) ~[spigot-1.11.2.jar:git-Spigot-7d78b81-27dd3ca]
	at net.minecraft.server.v1_11_R1.WorldServer.doTick(WorldServer.java:258) ~[spigot-1.11.2.jar:git-Spigot-7d78b81-27dd3ca]
	at net.minecraft.server.v1_11_R1.MinecraftServer.D(MinecraftServer.java:790) ~[spigot-1.11.2.jar:git-Spigot-7d78b81-27dd3ca]
	... 4 more
[23:22:05] [Server thread/ERROR]: 	Cause of unexpected exception was
java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429) ~[?:1.8.0_40]
	at java.util.HashMap$KeyIterator.next(HashMap.java:1453) ~[?:1.8.0_40]
	at net.minecraft.server.v1_11_R1.PlayerChunkMap.flush(PlayerChunkMap.java:122) ~[spigot-1.11.2.jar:git-Spigot-7d78b81-27dd3ca]
	at net.minecraft.server.v1_11_R1.WorldServer.doTick(WorldServer.java:258) ~[spigot-1.11.2.jar:git-Spigot-7d78b81-27dd3ca]
	at net.minecraft.server.v1_11_R1.MinecraftServer.D(MinecraftServer.java:790) ~[spigot-1.11.2.jar:git-Spigot-7d78b81-27dd3ca]
	at net.minecraft.server.v1_11_R1.DedicatedServer.D(DedicatedServer.java:399) ~[spigot-1.11.2.jar:git-Spigot-7d78b81-27dd3ca]
	at net.minecraft.server.v1_11_R1.MinecraftServer.C(MinecraftServer.java:675) ~[spigot-1.11.2.jar:git-Spigot-7d78b81-27dd3ca]
	at net.minecraft.server.v1_11_R1.MinecraftServer.run(MinecraftServer.java:574) [spigot-1.11.2.jar:git-Spigot-7d78b81-27dd3ca]
	at java.lang.Thread.run(Thread.java:745) [?:1.8.0_40]

This will occur whether Skyoseasons is the only loaded plugin or not.  Very rarely will I be able to change a season once.

Furthermore, from starting to work on my server again it has degenerated to the point where ice will not spawn and snow will either fall in patches or not at all, even on a completely fresh server.  I cannot comprehend why this has devolved as such.  I've checked permissions.  I'll let it generate new configs.  I'm stumped.  Still the same console outputs.

Occasionally, this will come up as well.

[23:20:59] [Server thread/INFO]: [Skyoseasons] [Suika] Autumn has come!
[23:21:00] [Thread-28/WARN]: Exception in thread "Thread-28" 
[23:21:00] [Thread-28/WARN]: java.lang.IllegalStateException: Asynchronous entity track!
[23:21:00] [Thread-28/WARN]: 	at org.spigotmc.AsyncCatcher.catchOp(AsyncCatcher.java:14)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.EntityTracker.addEntity(EntityTracker.java:108)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.EntityTracker.track(EntityTracker.java:66)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.WorldManager.a(WorldManager.java:21)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.World.b(World.java:1089)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.WorldServer.b(WorldServer.java:1112)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.WorldServer.a(WorldServer.java:1077)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.Chunk.addEntities(Chunk.java:753)
[23:21:00] [Thread-28/WARN]: 	at org.bukkit.craftbukkit.v1_11_R1.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:45)
[23:21:00] [Thread-28/WARN]: 	at org.bukkit.craftbukkit.v1_11_R1.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:1)
[23:21:00] [Thread-28/WARN]: 	at org.bukkit.craftbukkit.v1_11_R1.util.AsynchronousExecutor.skipQueue(AsynchronousExecutor.java:337)
[23:21:00] [Thread-28/WARN]: 	at org.bukkit.craftbukkit.v1_11_R1.util.AsynchronousExecutor.getSkipQueue(AsynchronousExecutor.java:295)
[23:21:00] [Thread-28/WARN]: 	at org.bukkit.craftbukkit.v1_11_R1.chunkio.ChunkIOExecutor.syncChunkLoad(ChunkIOExecutor.java:16)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:137)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:120)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:116)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.World.getChunkAt(World.java:329)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.ChunkCache.<init>(SourceFile:36)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.NavigationAbstract.a(SourceFile:109)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.Navigation.a(SourceFile:69)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.NavigationAbstract.j(SourceFile:78)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.NavigationListener.a(SourceFile:53)
[23:21:00] [Thread-28/WARN]: 	at net.minecraft.server.v1_11_R1.World.notify(World.java:460)
[23:21:00] [Thread-28/WARN]: 	at org.bukkit.craftbukkit.v1_11_R1.CraftWorld.refreshChunk(CraftWorld.java:248)
[23:21:00] [Thread-28/WARN]: 	at fr.skyost.seasons.SeasonWorld$2.run(SeasonWorld.java:309)

Running Spigot 1.11.2 with Skyoseasons 4.1.1.  Attempted both on Arch Linux and Windows 10 with ranges between 8-16 GB of memory allocated.

And of interesting note, this is what occurs in a new world without custom terrain generation, only running Skyoseasons, and all files in the server are made freshly.  You are looking towards spawn.