Weird problem with craftbukkit 1.6.4?

Whenever this one specific player tries to join my server, this error message comes up and he gets disconnected straight away...?

Code:

23.12 03:28:25 [Server] Startup §eTarzan1011011 left the game.
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.ThreadServerApplication.run(SourceFile:583)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.MinecraftServer.run(MinecraftServer.java:421)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.MinecraftServer.s(MinecraftServer.java:488)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.DedicatedServer.t(DedicatedServer.java:227)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.MinecraftServer.t(MinecraftServer.java:592)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.DedicatedServerConnection.b(SourceFile:30)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.ServerConnection.b(SourceFile:37)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.PlayerConnection.e(PlayerConnection.java:116)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.NetworkManager.b(NetworkManager.java:296)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.Packet10Flying.handle(SourceFile:136)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.PlayerConnection.a(PlayerConnection.java:419)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.EntityPlayer.b(EntityPlayer.java:492)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.EntityLiving.a(EntityLiving.java:136)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.Entity.a(Entity.java:771)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.EntityHuman.b(EntityHuman.java:1305)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.EntityLiving.b(EntityLiving.java:851)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.EntityPlayer.damageEntity(EntityPlayer.java:383)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.EntityHuman.damageEntity(EntityHuman.java:714)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.EntityLiving.damageEntity(EntityLiving.java:658)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.EntityHuman.d(EntityHuman.java:777)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.EntityLiving.c(EntityLiving.java:913)
23.12 03:28:25 [Server] Startup at net.minecraft.server.v1_6_R3.EnchantmentManager.a(SourceFile:144)
23.12 03:28:25 [Server] Startup at java.util.Random.nextInt(Random.java:300)
23.12 03:28:25 [Server] INFO java.lang.IllegalArgumentException: n must be positive
23.12 03:28:25 [Server] Startup Failed to handle packet for Tarzan1011011/139.218.214.194: java.lang.IllegalArgumentException: n must be positive

 

Probably a corrupt user data, or they have some mods installed.

Try erasing their user file inside worldname/players