I'm going to copypaste what I posted on the Yogscast forums, because as of yet I have gotten no help:
I have a very complicated system which I use to manage my custom mods I put onto my YCP. This includes the configs. It's very hard to work with sometimes, but I always manage to get it working. Generally, I never post in forums or have to bug anyone else to help me. I troubleshoot on my own or Google it, but this time, neither have worked.
I've been using the pack since it came out, and only now I'm having issues. The only leads I have is that I removed the Aether II and Threaded Lighting, but since I have not been to the Aether yet in any of my worlds I don't know why it does this.
Onto the actual issue (finally), whenever I try to run the modpack, it crashes. It closes the launcher, does nothing for a second and then the launcher comes back up. The only thing I could get out of the ATlauncher console that I thought would help was this: java.util.ConcurrentModificati onException
If you need any other information, such as the Forge Crash reports (notepad crashes when I try to open them, what's up with that?) or everything the console says, just ask. Thanks.
EDIT: I have something new worth noting. It's not just the Modpack. It seems to be a problem with Minecraft Forge, or maybe some of the other mods I have installed."
I've done a bit more digging in the files and found that it's having issues with certain files (as far as I can tell), such as "commons-io" or something like that. However, the commons files seem to be part of vanilla Minecraft. I've ran a fresh vanilla minecraft without forge on the ATLauncher, and it runs fine. But even when I only have a few mods installed it crashes and spits out that error message. Once again, if any additional information is required, just ask. Thanks.
Rollback Post to RevisionRollBack
Because why not put the RPG test thing on my signature?