Your problem is already fixed probably, but for the people who are searching for a fix, follow my steps

1. Search for CMD and run it as administrator
2. Type in the bar: netsh winsock reset
3. It should be fixed!
4. If it still doesn't work, try contacting Mojang, the creators of Minecraft

I had the same issue. I debugged it and found it to be caused by what I assume is a version mismatch between class net.minecraftforge.fml.common.network.NetworkRegistry and enum net.minecraftforge.fml.relauncher.Side.

Side has 3 values but the NetworkRegistry code appears to assume there are only 2, CLIENT and SERVER.

I fixed this by taking a copy of the NetworkRegistry class, and editing it as follows:

In the private, parameterless constructor, add the following line:

channels.put(Side.BUKKIT, Maps.<String,FMLEmbeddedChannel>newConcurrentMap());

Code change

Then you just need to make sure that your version of the class in on the classpath before the original version (if you're using eclipse then it will be automatically). You also need to make sure that you use the exact same package name for your version of the class.

Yes, of course it runs fine in your IDE. Have you noticed that it has 1.8.jar under Libraries tab? That means the "Client" in your Eclipse project is using some files(assets.*) inside 1.8.jar. The Client project in your Eclipse isn't the original Minecraft source code. It is modified a bit by MCP scripts to allow you to launch Minecraft client properly in your Eclipse. You have to copy a lot of stuffs from 1.8.jar and I am sure that Minecraft Launcher can't launch it unless you modify your exported Jar file and *.json.

If you really want to have your own Minecraft.jar, below are the correct steps. After you've done your modifications to Minecraft, save all files then close your Eclipse. In the directory(folder) which contains eclipse directory, you should fine reobfuscate.bat or reobfuscate.sh. Use the .bat version if you are Windows user, and use .sh if you are MacOS or Linux user. After you run the file, you should see some files in reobf directory. The files in the directory are obfuscated files and can be copied to Minecraft.jar directly. Now, use WinRar or other tools to open Minecraft.jar(1.8.jar). Copy all the files in reobf directory to Minecraft.jar(and replace if the file exists). After you finished these steps, you've got your own Minecraft.jar!

Now you should create your own version profile in Minecraft Launcher.(Reference: here)

In your .minecraft/versions directory, you should put your own Minecraft.jar. Predict you named it MyCustomMinecraft.jar, you have to create a new directory called MyCustomMinecraft in .minecraft/versions. Now, copy MyCustomMinecraft.jar and paste it in MyCustomMinecraft directory. Then, copy 1.8.json in versions/1.8 directory and paste it in MyCustomMinecraft. Rename 1.8.json to MyCustomMinecraft.json. Open MyCustomMinecraft.json with any text editor, in line 2, you should see "id": "1.8",. change it to MyCustomMinecraft. Your line 2 should look like "id": "MyCustomMinecraft", now.

Open Minecraft Launcher. Edit Profile button > Use version: > you should see release MyCustomMinecraft in the list!

So there are a few things you have to fix here because this won't work well.

First of all you have to use setExecutor() instead of creating a new instance of it. This can be done by adding this into your onEnable() :

this.getCommand("mycommand").setExecutor(new CommandKit());

You will have to specify this command in your plugin.yml as well, here is some documentation about it : https://www.spigotmc.org/wiki/plugin-yml/

Then you can delete your `HelloCommand()´, you won't need it anymore with what you changed above.

With this your error should go away, let me know if you still have some errors coming up.