This Error was first got on my main modpack, I'll attach modlist if needed. Error was reproduced on modpack with just nether ores, gregtech and all needed addons (ic2 and COFH Core).
Crash is happening during the PostInitialization phase, right at the end, when it proceeds to Avaliable state. Crash Report.
Sorry for my bad English. Thanks.

Looks like duplicate of this. Sorry.

This error was introduced into the IC2 source by greg, he can fix it.

Guys in greg's thread sayed that it's nether ores issue in this post. Well, nether ores and gregtech will be bad to have both at the same time, but still, it's error. I'll reopen this issue just in case.

Quite easy to fix on this side.
https://github.com/skyboy/NetherOres/blob/master/src/powercrystals/netherores/ores/Ores.java#L263
change: "if (NetherOresCore.enableMaceratorRecipes.getBoolean(true) &&Loader.isModLoaded("IC2"))" into "if (NetherOresCore.enableMaceratorRecipes.getBoolean(true) &&Loader.isModLoaded("IC2"))&&(!Loader.isModLoaded("gregtech"))"