I made a capability for an extended reach which works great on singleplayer, but on servers/multiplayer it doesn't work at all as the capability returned using EntityPlayer#getCapability(Capability, EnumFacing) returns null. After using some basic reflection I found out this is probably because the Entity#capabilities field is null, even though this is not the case on singleplayer.

CommonEventHandler.java (registered on both the client and the server):

package com.ptsmods.morecommands.miscellaneous;

import com.ptsmods.morecommands.miscellaneous.Reference.LogType;

import net.minecraft.entity.Entity;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.entity.player.EntityPlayerMP;
import net.minecraft.util.ResourceLocation;
import net.minecraftforge.event.AttachCapabilitiesEvent;
import net.minecraftforge.event.entity.player.PlayerEvent;
import net.minecraftforge.fml.common.FMLCommonHandler;
import net.minecraftforge.fml.common.eventhandler.SubscribeEvent;
import net.minecraftforge.fml.relauncher.Side;

public class CommonEventHandler extends EventHandler {

	public CommonEventHandler() {}

	@SubscribeEvent
	public void attachCapabilities(AttachCapabilitiesEvent<Entity> event) {
		if (event.getObject() instanceof EntityPlayer) Reference.print(LogType.INFO, "Attaching capability to player");
		if (!event.getCapabilities().containsKey(new ResourceLocation(Reference.MOD_ID, "reach")) && event.getObject() instanceof EntityPlayer) event.addCapability(new ResourceLocation(Reference.MOD_ID, "reach"), new ReachProvider());
	}

	@SubscribeEvent
	public void onPlayerClone(PlayerEvent.Clone event) {
		Reference.print(LogType.INFO, "Changed world, cloning reach capability.", event.getOriginal().getCapability(ReachProvider.reachCap, null).get(), FMLCommonHandler.instance().getSide(), Reference.isSingleplayer());
		event.getEntityPlayer().getCapability(ReachProvider.reachCap, null).set(FMLCommonHandler.instance().getSide() == Side.SERVER || Reference.isSingleplayer() ? (EntityPlayerMP) event.getEntityPlayer() : null, event.getOriginal().getCapability(ReachProvider.reachCap, null).get());
	}

}

IReach.java:

package com.ptsmods.morecommands.miscellaneous;

import javax.annotation.Nullable;

import net.minecraft.entity.player.EntityPlayerMP;

public interface IReach {

	public float get();

	public void set(float reach);

	public void set(@Nullable EntityPlayerMP player, float reach);

}

Reach.java:

package com.ptsmods.morecommands.miscellaneous;

import javax.annotation.Nullable;

import com.ptsmods.morecommands.net.ServerReachUpdatePacket;

import net.minecraft.entity.player.EntityPlayerMP;

public class Reach implements IReach {

	private float reach = 5;

	public Reach() {}

	@Override
	public float get() {
		return reach;
	}

	@Override
	public void set(float reach) {
		set(null, reach);
	}

	@Override
	public void set(@Nullable EntityPlayerMP player, float reach) {
		this.reach = reach < 1 ? 1F : reach;
		if (player != null) Reference.netWrapper.sendTo(new ServerReachUpdatePacket(reach), player);
	}

}

ReachStorage.java:

package com.ptsmods.morecommands.miscellaneous;

import net.minecraft.nbt.NBTBase;
import net.minecraft.nbt.NBTPrimitive;
import net.minecraft.nbt.NBTTagFloat;
import net.minecraft.util.EnumFacing;
import net.minecraftforge.common.capabilities.Capability;
import net.minecraftforge.common.capabilities.Capability.IStorage;

public class ReachStorage implements IStorage<IReach> {

	public ReachStorage() {}

	@Override
	public NBTBase writeNBT(Capability<IReach> capability, IReach instance, EnumFacing side) {
		return new NBTTagFloat(instance.get());
	}

	@Override
	public void readNBT(Capability<IReach> capability, IReach instance, EnumFacing side, NBTBase nbt) {
		instance.set(null, ((NBTPrimitive) nbt).getFloat());
	}

}

ReachProvider.java:

package com.ptsmods.morecommands.miscellaneous;

import net.minecraft.nbt.NBTBase;
import net.minecraft.util.EnumFacing;
import net.minecraftforge.common.capabilities.Capability;
import net.minecraftforge.common.capabilities.CapabilityInject;
import net.minecraftforge.common.capabilities.ICapabilitySerializable;

public class ReachProvider implements ICapabilitySerializable<NBTBase> {

	@CapabilityInject(IReach.class)
	public static final Capability<IReach>	reachCap	= null;
	private IReach							instance	= reachCap.getDefaultInstance();

	public ReachProvider() {}

	@Override
	public boolean hasCapability(Capability<?> capability, EnumFacing facing) {
		return capability == reachCap;
	}

	@Override
	public <T> T getCapability(Capability<T> capability, EnumFacing facing) {
		return capability == reachCap ? reachCap.<T>cast(instance) : null;
	}

	@Override
	public NBTBase serializeNBT() {
		return reachCap.getStorage().writeNBT(reachCap, instance, null);
	}

	@Override
	public void deserializeNBT(NBTBase nbt) {
		reachCap.getStorage().readNBT(reachCap, instance, null, nbt);
	}

}

And the capability is registered on the client in the preInit method and on the server in the serverStart method using the following line:

CapabilityManager.INSTANCE.register(IReach.class, new ReachStorage(), () -> new Reach());

Altough in the serverStart event it first checks to make sure it's not singleplayer.

Am I doing something wrong is this a collection of some internal problems? (Most likely the former)

I have fixed it myself, it seems like having a common event handler is not such a bright idea. I have moved both methods in the CommonEventHandler class to the ServerEventHandler and the ClientEventHandler and it now works fine.