I am pretty sure this is the Backpacks! mod that is causing this error but wanted to make you aware of it too. As the Backpacks! mod has no Github I have contact the dev via email as suggested by the Backpacks! dev. That email is: [email protected] for reference.

Here is my client crash log:

http://pastebin.com/H4qeceEz

Link to Backpacks! on Curse:

http://mods.curse.com/mc-mods/minecraft/223957-backpacks

Not wanting to point blame at anyone, but after hearing from the Backpacks! Dev, he stated:

I consider this to be Quark's problem, and it should be easy for them to fix on their end.
In fact, the following amendments should fix the problem

private static ModelBiped getPlayerArmorModel(AbstractClientPlayer player) {
List list = ReflectionHelper.getPrivateValue(RenderLivingBase.class, getRenderPlayer(player), LibObfuscation.LAYER_RENDERERS);
for (int i = 0; i < list.size(); i++) {
if (list.get(i) instanceof LayerBipedArmor) {
return ReflectionHelper.getPrivateValue(LayerArmorBase.class, (LayerArmorBase) list.get(i), LibObfuscation.MODEL_ARMOR);
}
}
return null;
}

private static ModelBiped getPlayerArmorLegModel(AbstractClientPlayer player) {
List list = ReflectionHelper.getPrivateValue(RenderLivingBase.class, getRenderPlayer(player), LibObfuscation.LAYER_RENDERERS);
for (int i = 0; i < list.size(); i++) {
if (list.get(i) instanceof LayerBipedArmor) {
return ReflectionHelper.getPrivateValue(LayerArmorBase.class, (LayerArmorBase) list.get(i), LibObfuscation.MODEL_LEGGINGS);
}
}
return null;
}

Awesome thank you.