So i'm trying to make my mod universal (ssp + smp) and i changed ModLoader.AddArmor to RenderingRegistry.addNewArmourRendererPrefix it works fine on ssp but i'm always getting this error on the server

 

[sEVERE] Encountered an unexpected exception LoaderException
cpw.mods.fml.common.LoaderException: java.lang.NoClassDefFoundError: net/minecraft/src/RenderPlayer
at cpw.mods.fml.common.LoadController.transition(LoadController.java:117)
at cpw.mods.fml.common.Loader.initializeMods(Loader.java:652)
at cpw.mods.fml.server.FMLServerHandler.finishServerLoading(FMLServerHandler.java:88)
at cpw.mods.fml.common.FMLCommonHandler.onServerStarted(FMLCommonHandler.java:344)
at net.minecraft.src.DedicatedServer.startServer(DedicatedServer.java:110)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:454)
at net.minecraft.src.ThreadMinecraftServer.run(ThreadMinecraftServer.java:17)
Caused by: java.lang.NoClassDefFoundError: net/minecraft/src/RenderPlayer
at cpw.mods.fml.client.registry.RenderingRegistry.addNewArmourRendererPrefix(RenderingRegistry.java:43)
at Obsidian.common.mod_Obsidian.load(mod_Obsidian.java:54)
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 cpw.mods.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:440)
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 com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:69)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:317)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:300)
at com.google.common.eventbus.EventBus.post(EventBus.java:268)
at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:140)
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 com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:69)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:317)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:300)
at com.google.common.eventbus.EventBus.post(EventBus.java:268)
at cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:83)
at cpw.mods.fml.common.Loader.initializeMods(Loader.java:651)
... 5 more
Caused by: java.lang.ClassNotFoundException: net.minecraft.src.RenderPlayer
at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:126)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 33 more
Caused by: java.lang.RuntimeException: Attempted to load class net/minecraft/src/RenderPlayer for invalid side SERVER
at cpw.mods.fml.common.asm.transformers.SideTransformer.transform(SideTransformer.java:38)
at cpw.mods.fml.relauncher.RelaunchClassLoader.runTransformers(RelaunchClassLoader.java:163)
at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:119)
... 35 more

You're trying to use client-side code on the server-side. You need to use a SidedProxy.

I don't know how to use proxy's for armor...

Forge has an interface for this. Look at IArmorTextureProvider for a solution