Server crashes with Java Exception shortly after logging in

Logged in, behaviour normal for about 20 seconds, then server crashes with the following log:

[13:13:58] [Server thread/ERROR]: Encountered an unexpected exception
e: Exception ticking world
at net.minecraft.server.MinecraftServer.D(SourceFile:621) ~[minecraft_server.15w49a.jar:?]
at la.D(SourceFile:339) ~[minecraft_server.15w49a.jar:?]
at net.minecraft.server.MinecraftServer.C(SourceFile:553) ~[minecraft_server.15w49a.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:457) [minecraft_server.15w49a.jar:?]
at java.lang.Thread.run(Thread.java:745) [?:1.8.0_66]
Caused by: java.lang.IllegalArgumentException: Cannot get property arh{name=half, clazz=class als$a, values=[top, bottom]} as it does not exist in aqv{block=minecraft:double_wooden_slab, properties=[variant]}
at aqv$a.c(SourceFile:165) ~[minecraft_server.15w49a.jar:?]
at als.k(SourceFile:50) ~[minecraft_server.15w49a.jar:?]
at aqv$a.q(SourceFile:388) ~[minecraft_server.15w49a.jar:?]
at ahu.a(SourceFile:203) ~[minecraft_server.15w49a.jar:?]
at ahu.a(SourceFile:132) ~[minecraft_server.15w49a.jar:?]
at lp.d(SourceFile:198) ~[minecraft_server.15w49a.jar:?]
at net.minecraft.server.MinecraftServer.D(SourceFile:617) ~[minecraft_server.15w49a.jar:?]
... 4 more
[13:13:58] [Server thread/ERROR]: This crash report has been saved to: /opt/msm/servers/combatdvorak/./crash-reports/crash-2015-12-03_13.13.58-server.txt
[13:13:58] [Server thread/INFO]: Stopping server
[13:13:58] [Server thread/INFO]: Saving players
[13:13:58] [Server thread/INFO]: Saving worlds
[13:13:58] [Server thread/INFO]: Saving chunks for level 'dvorak'/Overworld
[13:13:58] [Server thread/INFO]: Saving chunks for level 'dvorak'/Nether
[13:13:58] [Server thread/INFO]: Saving chunks for level 'dvorak'/The End