Exception on SMP server execution

My private server crashed during normal execution. Please see the crash log below. I hope this contains all the information you need. As far as I can see there was one player online, but I don't know rigth now what he was doing at the time of the crash.

---- Minecraft Crash Report ----
// Surprise! Haha. Well, this is awkward.

Time: 3/19/13 10:12 PM
Description: Exception ticking world

java.lang.NullPointerException
	at iy.b(SourceFile:199)
	at iz.b(SourceFile:122)
	at net.minecraft.server.MinecraftServer.r(SourceFile:454)
	at hz.r(SourceFile:255)
	at net.minecraft.server.MinecraftServer.q(SourceFile:397)
	at net.minecraft.server.MinecraftServer.run(SourceFile:331)
	at gp.run(SourceFile:573)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
	at iy.b(SourceFile:199)
	at iz.b(SourceFile:122)

-- Affected level --
Details:
	Level name: thekeep
	All players: 0 total; []
	Chunk stats: ServerChunkCache: 302 Drop: 304
	Level seed: -7358406435562637852
	Level generator: ID 00 - default, ver 1. Features enabled: false
	Level generator options: 
	Level spawn location: World: (0,0,0), Chunk: (at 0,0,0 in 0,0; contains blocks 0,0,0 to 15,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
	Level time: 0 game time, 0 day time
	Level dimension: 0
	Level storage version: 0x00000 - Unknown?
	Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
	at net.minecraft.server.MinecraftServer.r(SourceFile:454)
	at hz.r(SourceFile:255)
	at net.minecraft.server.MinecraftServer.q(SourceFile:397)
	at net.minecraft.server.MinecraftServer.run(SourceFile:331)
	at gp.run(SourceFile:573)

-- System Details --
Details:
	Minecraft Version: 1.5
	Operating System: Linux (amd64) version 3.2.13-grsec-xxxx-grs-ipv6-64
	Java Version: 1.7.0_03, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 406680640 bytes (387 MB) / 518979584 bytes (494 MB) up to 518979584 bytes (494 MB)
	JVM Flags: 4 total; -XX:+AggressiveOpts -XX:ParallelGCThreads=3 -Xmx512M -Xincgc
	AABB Pool Size: 3857 (215992 bytes; 0 MB) allocated, 3783 (211848 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 1, tallocated: 63
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 890 (49840 bytes; 0 MB) allocated, 819 (45864 bytes; 0 MB) used
	Player Count: 1 / 10; [jc['--redacted--'/8374, l='thekeep', x=7.50, y=29.10, z=284.50]]
	Is Modded: Unknown (can't tell)
	Type: Dedicated Server (map_server.txt)