This thing is happening for me too. I've spent most of the day tracking down what i thought was a specific mod that errored. But now its clear that its just the number of mods/IDs added that really counts. I got all my mods added to the client (62 ATM) without a problem, and i can even start a SP world without any conflicts. The problem occurs when i want to join my dedicated server.
If i add a mod with a lot of IDs etc. Forestry or Natura, I have to remove a lot of the "smaller" mods like jabba/enderstorage/RemoteIO.
I use Forge 10.13.0.1180 on both client and local server installed on windows 7 ultimate x64 with java 7_60
