2012-02-13 22:07:09 [SEVERE] null
org.bukkit.command.CommandException: Unhandled exception executing command 'vanish' in plugin VanishNoPacket v3.1
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:42)
at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:168)
at org.bukkit.craftbukkit.CraftServer.dispatchCommand(CraftServer.java:386)
at net.minecraft.server.NetServerHandler.handleCommand(NetServerHandler.java:777)
at net.minecraft.server.NetServerHandler.chat(NetServerHandler.java:737)
at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:725)
at org.getspout.spout.SpoutNetServerHandler.a(SpoutNetServerHandler.java:176)
at net.minecraft.server.Packet3Chat.handle(Packet3Chat.java:33)
at net.minecraft.server.NetworkManager.b(NetworkManager.java:226)
at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:100)
at org.getspout.spout.SpoutNetServerHandler.a(SpoutNetServerHandler.java:552)
at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:78)
at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:537)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:435)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:465)
Caused by: java.lang.NoSuchMethodError: org.bukkit.entity.Player.canSee(Lorg/bukkit/entity/Player;)Z
at org.kitteh.vanish.VanishManager.toggleVanishQuiet(VanishManager.java:200)
at org.kitteh.vanish.VanishManager.toggleVanish(VanishManager.java:142)
at org.kitteh.vanish.VanishCommand.onCommand(VanishCommand.java:43)
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:40)
... 14 more

2012-02-13 22:09:01 [SEVERE] Could not pass event org.bukkit.event.player.PlayerJoinEvent to VanishNoPacket
org.bukkit.event.EventException
at org.bukkit.plugin.java.JavaPluginLoader$103.execute(JavaPluginLoader.java:1026)
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:57)
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:453)
at net.minecraft.server.ServerConfigurationManager.c(ServerConfigurationManager.java:133)
at net.minecraft.server.NetLoginHandler.b(NetLoginHandler.java:121)
at net.minecraft.server.NetLoginHandler.a(NetLoginHandler.java:40)
at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:61)
at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:537)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:435)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:465)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.bukkit.plugin.java.JavaPluginLoader$103.execute(JavaPluginLoader.java:1024)
... 9 more
Caused by: java.lang.NoSuchMethodError: org.bukkit.entity.Player.canSee(Lorg/bukkit/entity/Player;)Z
at org.kitteh.vanish.VanishManager.showVanished(VanishManager.java:238)
at org.kitteh.vanish.VanishManager.resetSeeing(VanishManager.java:115)
at org.kitteh.vanish.listeners.ListenPlayerJoin.onPlayerJoinLate(ListenPlayerJoin.java:48)
... 14 more

bukkit version:1.1-R4 (b1938) (same problem with 1.1-R3)

vanishnopacket version: v3.1

If i´m the only person on the server it works fine, but if another one jons nothing is working.

When i type /vanish an internal error occoured.

Hope you can help me :)

I assure you that you are not getting that error on 1.1-R4. Upgrade to R4 for certain, and try again.

done, still getting the same problem

Start up your 1.1-R4 server. Type this into the console: version

2012-02-14 22:39:05 [INFO] This server is running CraftBukkit version git-Bukkit-1.1-R4-b1938jnks (MC: 1.1) (Implementing API version 1.1-R4)�[0m

That error says it can't find Player.canSee(Player)

Which is right here https://github.com/Bukkit/Bukkit/blob/master/src/main/java/org/bukkit/entity/Player.java#L514

Are you certain this exact error is happening on the 1.1 R4 server?