MyWorlds world loading error

[WARNING] [My Worlds] World load issue: Failed to check session lock, aborting
Error: java.io.FileNotFoundException: ./world/session.lock (Permission denied)
    at org.bukkit.craftbukkit.CraftServer.createWorld(CraftServer.java:723)
    at net.minecraft.server.ServerNBTManager.<init>(SourceFile:16)
    at org.bukkit.craftbukkit.CraftServer.dispatchCommand(CraftServer.java:532)
    at net.minecraft.server.DedicatedServerConnection.b(SourceFile:30)
    at java.io.FileOutputStream.<init>(Unknown Source)
    at org.bukkit.WorldCreator.createWorld(WorldCreator.java:235)
    at java.io.FileOutputStream.<init>(Unknown Source)
    at net.minecraft.server.WorldNBTStorage.h(WorldNBTStorage.java:46)
    at net.minecraft.server.WorldNBTStorage.<init>(WorldNBTStorage.java:40)
    at com.bergerkiller.bukkit.mw.commands.Command.execute(Command.java:342)
    at net.minecraft.server.WorldNBTStorage.<init>(WorldNBTStorage.java:40)
    at java.io.FileOutputStream.open(Native Method)
    at com.earth2me.essentials.Essentials.onCommandEssentials(Essentials.java:314)
    at com.bergerkiller.bukkit.mw.MyWorlds.command(MyWorlds.java:154)
    at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44)
    at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44)
    at net.minecraft.server.DedicatedServerConnection.b(SourceFile:30)
    at com.earth2me.essentials.Essentials.onCommandEssentials(Essentials.java:314)
    at net.minecraft.server.DedicatedServer.r(DedicatedServer.java:222)
    at org.bukkit.craftbukkit.CraftServer.createWorld(CraftServer.java:723)
    at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:186)
    at org.bukkit.craftbukkit.CraftServer.dispatchCommand(CraftServer.java:532)
    at org.bukkit.Bukkit.createWorld(Bukkit.java:155)
    at net.minecraft.server.NetServerHandler.handleCommand(NetServerHandler.java:998)
    at com.earth2me.essentials.Essentials.onCommand(Essentials.java:299)
    at net.minecraft.server.NetServerHandler.chat(NetServerHandler.java:914)
    at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:858)
[WARNING] [My Worlds] Operation failed after 4 retries!
    at org.bukkit.craftbukkit.CraftServer.createWorld(CraftServer.java:723)
    at com.bergerkiller.bukkit.mw.WorldManager.createWorld(WorldManager.java:426)
    at net.minecraft.server.NetworkManager.b(NetworkManager.java:290)
    at org.bukkit.Bukkit.createWorld(Bukkit.java:155)
    at net.minecraft.server.ServerNBTManager.<init>(SourceFile:16)
    at org.bukkit.WorldCreator.createWorld(WorldCreator.java:235)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:856)
[WARNING] [My Worlds] World load issue: Failed to check session lock, aborting
    at net.minecraft.server.NetServerHandler.d(NetServerHandler.java:113)
    at com.bergerkiller.bukkit.mw.commands.WorldLoad.execute(WorldLoad.java:23)
    at net.minecraft.server.Packet3Chat.handle(Packet3Chat.java:44)
    at net.minecraft.server.MinecraftServer.r(MinecraftServer.java:596)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:426)
    at com.bergerkiller.bukkit.common.PluginBase.onCommand(PluginBase.java:573)
    at net.minecraft.server.ServerConnection.b(SourceFile:39)
    at org.bukkit.WorldCreator.createWorld(WorldCreator.java:235)
    at net.minecraft.server.WorldNBTStorage.h(WorldNBTStorage.java:55)
    at net.minecraft.server.WorldNBTStorage.<init>(WorldNBTStorage.java:40)
    at net.minecraft.server.MinecraftServer.q(MinecraftServer.java:494)