java.lang.NullPointerException: Ticking entity (Entity Type: Zombie)

Playing from Mac OS X client on my world running on a server box on my local network. The server is running Linux (see Environment above). I started to attack a zombie when I noticed that everything stopped moving and the animal sounds stopped. I disconnected and tried to reconnect but only got timeout errors. I connected to the Minecraft server via ssh and used my control script to stop and start the server again. After that the connection would be refused. The first error is listed in the summary of this issue. 60 seconds later, the server logged another error of type java.lang.Thread.sleep(Native Method). I have attached both crash reports.

First crash:

java.lang.NullPointerException: Ticking entity
	at bou.a(SourceFile:77)
	at bou.a(SourceFile:65)
	at boy.a(SourceFile:77)
	at boy.a(SourceFile:37)
	at boy.a(SourceFile:27)
	at aay.a(SourceFile:71)
	at zl.a(SourceFile:57)
	at zb.a(SourceFile:72)
	at xl.bE(SourceFile:446)
	at xk.l(SourceFile:1430)
	at xl.l(SourceFile:316)
	at afc.l(SourceFile:39)
	at afz.l(SourceFile:203)
	at xk.j(SourceFile:1298)
	at xl.j(SourceFile:194)
	at afc.j(SourceFile:44)
	at afz.j(SourceFile:259)
	at aqh.a(SourceFile:1410)
	at qo.a(SourceFile:601)
	at aqh.g(SourceFile:1388)
	at aqh.i(SourceFile:1281)
	at qo.i(SourceFile:479)
	at net.minecraft.server.MinecraftServer.y(SourceFile:606)
	at pk.y(SourceFile:303)
	at net.minecraft.server.MinecraftServer.x(SourceFile:530)
	at net.minecraft.server.MinecraftServer.run(SourceFile:446)
	at java.lang.Thread.run(Thread.java:745)

First crash detail:

---- Minecraft Crash Report ----
// Daisy, daisy...

Time: 7/30/14 11:09 PM
Description: Ticking entity

java.lang.NullPointerException: Ticking entity
	at bou.a(SourceFile:77)
	at bou.a(SourceFile:65)
	at boy.a(SourceFile:77)
	at boy.a(SourceFile:37)
	at boy.a(SourceFile:27)
	at aay.a(SourceFile:71)
	at zl.a(SourceFile:57)
	at zb.a(SourceFile:72)
	at xl.bE(SourceFile:446)
	at xk.l(SourceFile:1430)
	at xl.l(SourceFile:316)
	at afc.l(SourceFile:39)
	at afz.l(SourceFile:203)
	at xk.j(SourceFile:1298)
	at xl.j(SourceFile:194)
	at afc.j(SourceFile:44)
	at afz.j(SourceFile:259)
	at aqh.a(SourceFile:1410)
	at qo.a(SourceFile:601)
	at aqh.g(SourceFile:1388)
	at aqh.i(SourceFile:1281)
	at qo.i(SourceFile:479)
	at net.minecraft.server.MinecraftServer.y(SourceFile:606)
	at pk.y(SourceFile:303)
	at net.minecraft.server.MinecraftServer.x(SourceFile:530)
	at net.minecraft.server.MinecraftServer.run(SourceFile:446)
	at java.lang.Thread.run(Thread.java:745)


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

-- Head --
Stacktrace:
	at bou.a(SourceFile:77)
	at bou.a(SourceFile:65)
	at boy.a(SourceFile:77)
	at boy.a(SourceFile:37)
	at boy.a(SourceFile:27)
	at aay.a(SourceFile:71)
	at zl.a(SourceFile:57)
	at zb.a(SourceFile:72)
	at xl.bE(SourceFile:446)
	at xk.l(SourceFile:1430)
	at xl.l(SourceFile:316)
	at afc.l(SourceFile:39)
	at afz.l(SourceFile:203)
	at xk.j(SourceFile:1298)
	at xl.j(SourceFile:194)
	at afc.j(SourceFile:44)
	at afz.j(SourceFile:259)
	at aqh.a(SourceFile:1410)
	at qo.a(SourceFile:601)
	at aqh.g(SourceFile:1388)

-- Entity being ticked --
Details:
	Entity Type: Zombie (afz)
	Entity ID: 268763
	Entity Name: Zombie
	Entity's Exact location: 123.50, 63.00, 1534.22
	Entity's Block location: 123.00,63.00,1534.00 - World: (123,63,1534), Chunk: (at 11,3,14 in 7,95; contains blocks 112,0,1520 to 127,255,1535), Region: (0,2; contains chunks 0,64 to 31,95, blocks 0,0,1024 to 511,255,1535)
	Entity's Momentum: -0.31, 0.36, -0.25
	Entity's Rider: ~~ERROR~~ NullPointerException: null
	Entity's Vehicle: ~~ERROR~~ NullPointerException: null
Stacktrace:
	at aqh.i(SourceFile:1281)
	at qo.i(SourceFile:479)

-- Affected level --
Details:
	Level name: Furya
	All players: 1 total; [qr['danielmcq'/163, l='Furya', x=125.01, y=63.00, z=1535.46]]
	Chunk stats: ServerChunkCache: 1592 Drop: 0
	Level seed: 973714163
	Level generator: ID 00 - default, ver 1. Features enabled: true
	Level generator options: 
	Level spawn location: 8.00,64.00,-28.00 - World: (8,64,-28), Chunk: (at 8,4,4 in 0,-2; contains blocks 0,0,-32 to 15,255,-17), Region: (0,-1; contains chunks 0,-32 to 31,-1, blocks 0,0,-512 to 511,255,-1)
	Level time: 174777729 game time, 5826720 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 4581 (now: true), thunder time: 99128 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
	at net.minecraft.server.MinecraftServer.y(SourceFile:606)
	at pk.y(SourceFile:303)
	at net.minecraft.server.MinecraftServer.x(SourceFile:530)
	at net.minecraft.server.MinecraftServer.run(SourceFile:446)
	at java.lang.Thread.run(Thread.java:745)

-- System Details --
Details:
	Minecraft Version: 14w31a
	Operating System: Linux (amd64) version 3.2.0-4-amd64
	Java Version: 1.7.0_65, Oracle Corporation
	Java VM Version: OpenJDK 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 880212888 bytes (839 MB) / 1215299584 bytes (1159 MB) up to 1431830528 bytes (1365 MB)
	JVM Flags: 2 total; -Xmx1536M -Xms1024M
	IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
	Profiler Position: N/A (disabled)
	Player Count: 1 / 5; [qr['danielmcq'/163, l='Furya', x=125.01, y=63.00, z=1535.46]]
	Is Modded: Unknown (can't tell)
	Type: Dedicated Server (map_server.txt)