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.