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I'm running a vanilla 1.8.1 Minecraft server on an Ubuntu Server. I'm using a whitelist, so the number of distinct players is low. The server has been running for about six weeks but the world is actually much older. It comes from the files of a friend's old, discontinued server.

I am experiencing crashes at a rate of around one per day, and I have just received my first crash report from Minecraft. When Minecraft crashes, users are disconnected and the server saves its data and stops. There is not an obvious pattern to when it crashes. The server itself remains operational. Minecraft can be subsequently restarted and the world can run for hours without another crash.

The crash report references a nullpointerexception involving a pigman at 0,17,0, I think in the Overworld(?). This block is three or four blocks above the floor, and exactly two blocks to the right of a nether portal (the border of the portal begins at 0,17,-2). There is a possibility (I am not sure) that someone has been near this portal so that it was loaded at the time of each crash. However, it is certain that most of the time the area is loaded no crash occurs.

Any help with understanding this error and knowing how to resolve it would be greatly appreciated. Thank you.

Ubuntu Startup Message (just so you get a little information about the server)

Welcome to Ubuntu 14.04.1 LTS (GNU/Linux 3.13.0-43-generic i686)

* Documentation: https://help.ubuntu.com/

System information as of Sat Jan 3 23:27:12 EST 2015

System load: 0.0 Processes: 82
Usage of /: 1.7% of 271.00GB Users logged in: 0
Memory usage: 2% IP address for eth0: 192.168.1.100
Swap usage: 0%

Graph this data and manage this system at:
https://landscape.canonical.com/

0 packages can be updated.
0 updates are security updates.

Minecraft Crash Report


---- Minecraft Crash Report ----
// Don't be sad, have a hug! <3

Time: 1/3/15 4:53 PM
Description: Ticking entity

java.lang.NullPointerException: Ticking entity


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

-- Head --
Stacktrace:
at aqr.i(SourceFile:1281)

-- Entity being ticked --
Details:
Entity Type: PigZombie (afn)
Entity ID: 4092710
Entity Name: Zombie Pigman
Entity's Exact location: 0.00, 17.50, 0.00
Entity's Block location: 0.00,17.00,0.00 - World: (0,17,0), Chunk: (at 0,1,0 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)
Entity's Momentum: 0.00, -0.08, -0.03
Entity's Rider: ~~ERROR~~ NullPointerException: null
Entity's Vehicle: ~~ERROR~~ NullPointerException: null
Stacktrace:
at aqr.i(SourceFile:1281)
at qu.i(SourceFile:478)
at net.minecraft.server.MinecraftServer.A(SourceFile:608)
at pp.A(SourceFile:299)
at net.minecraft.server.MinecraftServer.z(SourceFile:532)
at net.minecraft.server.MinecraftServer.run(SourceFile:448)
at java.lang.Thread.run(Thread.java:745)

-- Affected level --
Details:
Level name: Limbo
All players: 1 total; [qx['daemonten'/4087810, l='Limbo', x=-4.51, y=17.50, z=2.17]]
Chunk stats: ServerChunkCache: 687 Drop: 0
Level seed: -21246052028539666
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: -48.00,64.00,248.00 - World: (-48,64,248), Chunk: (at 0,4,8 in -3,15; contains blocks -48,0,240 to -33,255,255), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
Level time: 301788184 game time, 240974964 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 10270 (now: false), thunder time: 28393 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false

-- System Details --
Details:
Minecraft Version: 1.8.1
Operating System: Linux (i386) version 3.13.0-43-generic
Java Version: 1.7.0_65, Oracle Corporation
Java VM Version: OpenJDK Server VM (mixed mode), Oracle Corporation
Memory: 1168314128 bytes (1114 MB) / 2069102592 bytes (1973 MB) up to 2069102592 bytes (1973 MB)
JVM Flags: 2 total; -Xmx2G -Xms2G
IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
Profiler Position: N/A (disabled)
Player Count: 1 / 10; [qx['daemonten'/4087810, l='Limbo', x=-4.51, y=17.50, z=2.17]]
Is Modded: Unknown (can't tell)
Type: Dedicated Server (map_server.txt)

Thanks for your quick response! Can you tell me what part of the report indicates a corrupt entity, or how you can tell this is the case? I want to learn to fish a little, although I appreciate the fish you've given me.

The report references a pigman. Does this mean the corrupt entity is a pigman?

We think we figured it out. Several players were using stairs placed directly in front of their nether portals, leading to each side of the nether portal, to prevent pigmen from wandering out of the portals. This was never a problem until the new portal behavior of 1.8.1, but it seems pigmen were getting placed on the stairs that previously would have held them inside of the portal, causing the error. We've cleared the stairs and the problem seems to be resolved.

Sounds like a bug that should be posted to the bug tracker.