A detailed walkthrough of the error, its code path and all known details is as follows:
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-- Head --
Thread: Server thread
Stacktrace:
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)
	at java.util.HashMap$KeyIterator.next(HashMap.java:1469)
	at net.minecraft.server.management.PlayerChunkMap.func_72693_b(SourceFile:114)
	at net.minecraft.world.WorldServer.func_72835_b(WorldServer.java:227)

-- Affected level --
Details:
	Level name: world
	All players: 2 total; [EntityPlayerMP['Machiko98'/1980235, l='world', x=-626.89, y=66.00, z=202.28], EntityPlayerMP['Forgrim11'/1980866, l='world', x=-626.87, y=66.00, z=200.00]]
	Chunk stats: ServerChunkCache: 697 Drop: 0
	Level seed: -4644884349389603545
	Level generator: ID 06 - BIOMESOP, ver 0. Features enabled: true
	Level generator options:
	Level spawn location: World: (-140,64,256), Chunk: (at 4,4,0 in -9,16; contains blocks -144,0,256 to -129,255,271), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
	Level time: 7205068 game time, 7508232 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 72055 (now: false), thunder time: 52228 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
	at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:756)
	at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:397)
	at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:668)
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526)
	at java.lang.Thread.run(Thread.java:748)