Could not load plugin

Hi!
I have serious problems..
I can't load ANY Plugin!
I did no changes to any settings.. I just restarted my server.
I currently have only 1 plugin in my plugins folder. but whatever plugin I move there, it can't load!

Code:

Could not load 'plugins/WorldGuard.jar' in folder 'plugins':
java.lang.NoClassDefFoundError: com/sk89q/minecraft/util/commands/CommandException
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:264)
        at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:171)
        at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:215)
        at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:136)
        at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:155)
        at org.bukkit.craftbukkit.CraftServer.<init>(CraftServer.java:131)
        at net.minecraft.server.ServerConfigurationManager.<init>(ServerConfigurationManager.java:52)
        at net.minecraft.server.MinecraftServer.init(MinecraftServer.java:147)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:406)
        at net.minecraft.server.ThreadServerApplication.run(SourceFile:465)
Caused by: java.lang.ClassNotFoundException: com.sk89q.minecraft.util.commands.CommandException
        at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
        at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:36)
        at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:24)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
        at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:334)
        ... 11 more