I set up a small IC^2 server for me and a few friends, but every so often when somebody places a block everyone on the server crashes with
It also seems to persist in one area for awhile then move elsewhere; any blocks placed in the area will most likely crash the game. The mods i have running are:
Client:
AudioMod
Buildcraft v1.8.1.1
IndustrialCraft^2 v1.15
Minecraft Forge v1.1.1
ModLoaderModLoaderMP
Player's ModLoaderMP Client Fix
Player's SoundSystem Update
Server:
Buildcraft v1.8.1.1
IndustrialCraft^2 v1.15
Minecraft Forge 1.1.1
ModloaderMP
Player's ModLoaderMP Server Fix
Flan's MP Utils (Crashes occured before installing as well)
I also used to have Flan's WWII Guns v3, Flan's Vehicles v4, Flan's Planes v16.1, Gary's TurboModelThingy v2.3.4, and PlayerAPI v1, as well as their respective server versions. All of those are, at this point, uninstalled.
If it matters I also upgraded the world using IndustrialUpgrade (Going from 1.7.3 with IC 8.55_02 to 1.8.1 with IC^2 1.15) and even though it says only SSP worlds are supported, i see no reason why it should cause an error such as this (however I know very little about Java). After discovering the problem I then ran Chunkster (to no avail)
I also have no idea which mod specifically is causing the problem (or, however unlikely it may be, the vanilla software), but I decided to post here anyway (a quick forum search turned up nothing).
Running Java JDK 1.7 64bit
Code tags not cooperating 
EDIT: Appears to happen when a machine is in use NOPE
