When 4 players write the command /join the server have a error:
My server is 1.6.4

2013-12-23 03:25:39 [INFO] Riotineveryone issued server command: /join
2013-12-23 03:25:43 [INFO] [COMMAND] LastWishSpain: /join
2013-12-23 03:25:43 [INFO] LastWishSpain issued server command: /join
2013-12-23 03:25:48 [INFO] [COMMAND] LastWishSpain: /lobby
2013-12-23 03:25:48 [INFO] LastWishSpain issued server command: /lobby
2013-12-23 03:25:51 [INFO] [COMMAND] KrissLepard: /join
2013-12-23 03:25:51 [INFO] KrissLepard issued server command: /join
2013-12-23 03:25:51 [SEVERE] null
org.bukkit.command.CommandException: Unhandled exception executing command 'join' in plugin TheWalls v2.1
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46)
at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:192)
at org.bukkit.craftbukkit.v1_6_R3.CraftServer.dispatchCommand(CraftServer.java:528)
at net.minecraft.server.v1_6_R3.PlayerConnection.handleCommand(PlayerConnection.java:968)
at net.minecraft.server.v1_6_R3.PlayerConnection.chat(PlayerConnection.java:886)
at net.minecraft.server.v1_6_R3.PlayerConnection.a(PlayerConnection.java:837)
at net.minecraft.server.v1_6_R3.Packet3Chat.handle(SourceFile:49)
at net.minecraft.server.v1_6_R3.NetworkManager.b(NetworkManager.java:296)
at net.minecraft.server.v1_6_R3.PlayerConnection.e(PlayerConnection.java:116)
at net.minecraft.server.v1_6_R3.ServerConnection.b(SourceFile:37)
at net.minecraft.server.v1_6_R3.DedicatedServerConnection.b(SourceFile:30)
at net.minecraft.server.v1_6_R3.MinecraftServer.t(MinecraftServer.java:592)
at net.minecraft.server.v1_6_R3.DedicatedServer.t(DedicatedServer.java:227)
at net.minecraft.server.v1_6_R3.MinecraftServer.s(MinecraftServer.java:488)
at net.minecraft.server.v1_6_R3.MinecraftServer.run(MinecraftServer.java:421)
at net.minecraft.server.v1_6_R3.ThreadServerApplication.run(SourceFile:583)
Caused by: org.bukkit.plugin.IllegalPluginAccessException: Plugin attempted to register task while disabled
at org.bukkit.craftbukkit.v1_6_R3.scheduler.CraftScheduler.validate(CraftScheduler.java:394)
at org.bukkit.craftbukkit.v1_6_R3.scheduler.CraftScheduler.runTaskTimer(CraftScheduler.java:120)
at org.bukkit.craftbukkit.v1_6_R3.scheduler.CraftScheduler.scheduleSyncRepeatingTask(CraftScheduler.java:116)
at me.NerdsWBNerds.TheWalls.TheWalls.startQueCount(TheWalls.java:678)
at me.NerdsWBNerds.TheWalls.TheWalls.checkQue(TheWalls.java:668)
at me.NerdsWBNerds.TheWalls.TheWalls.addPlayer(TheWalls.java:607)
at me.NerdsWBNerds.TheWalls.TheWalls.addPlayer(TheWalls.java:614)
at me.NerdsWBNerds.TheWalls.Commands.CommandHub.joinGame(CommandHub.java:207)
at me.NerdsWBNerds.TheWalls.Commands.GeneralCMD.onCommand(GeneralCMD.java:36)
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44)
... 15 more