I am getting the same error on my server but only when I load my current world, it doesn't do this on a new world.
Minecraft Server:1.2.5
Minecraft Client:1.2.5
Modloader 1.2.5
IC2: 1.95
MCForge_3.1.2.97 Client and Server
Code
java.lang.NegativeArraySizeException
at ic2.platform.NetworkManager.onPacketData(NetworkManager.java:205)
at forge.MessageManager.dispatchIncomingMessage(MessageManager.java:432)
at adl.a(NetClientHandler.java:1246)
at ee.a(SourceFile:46)
at lg.b(NetworkManager.java:343)
at adl.b(NetClientHandler.java:68)
at je.c(SourceFile:50)
at net.minecraft.client.Minecraft.k(Minecraft.java:1867)
at net.minecraft.client.Minecraft.x(Minecraft.java:819)
at net.minecraft.client.Minecraft.run(Minecraft.java:747)
at java.lang.Thread.run(Unknown Source)
Stopping!