Here is the crash:

---- Minecraft Crash Report ----
// I just don't know what went wrong

Time: 31/07/14 23:08
Description: Exception in server tick loop

java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
at java.util.HashMap$KeyIterator.next(Unknown Source)
at qi.a(SourceFile:163)
at net.minecraft.server.MinecraftServer.y(SourceFile:614)
at net.minecraft.server.MinecraftServer.x(SourceFile:530)
at cxc.x(SourceFile:155)
at net.minecraft.server.MinecraftServer.run(SourceFile:446)
at java.lang.Thread.run(Unknown Source)

A detailed walkthrough of the error, its code path and all known details is as follows:
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– System Details –
Details:
Minecraft Version: 14w31a
Operating System: Windows 8 (amd64) version 6.2
Java Version: 1.7.0_51, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 226450592 bytes (215 MB) / 486842368 bytes (464 MB) up to 1060372480 bytes (1011 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 95
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [qr['TheTalkingWhale'/333, l='0013', x=-808.16, y=12.00, z=598.68]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.