So, this is new today. I've reinstalled all my Java versions to no avail. Don't know what happened, don't know enough about Java to understand what's happening. :C Vanilla MC works fine, but modded does not, and that includes with other launchers as well. http://pastebin.com/Bbj0bQrx
I've looked into it and this seems to only be an issue while dealing with Java 8. Have you tried using Java 6 or 7 instead? Note that even if Java 6, 7, and 8 are all installed, the launcher will usually use the most recent version of Java.
In order to help, I need to know OS and such, as well as exact Java version. After that, it might take me a few days to get back to you on a fix. If using Windows 8/8.1, use Java ; as compatibility issues come up.
As far as I know, if you have Java 8 installed on a PC, Minecraft will attempt using that version of java. That will probably mean that the mods installed will also attempt using that Java version; if their coding is no longer supported in Java 8, then chances are that you will crash (like this). Although I'm unsure how to select which Java version to use on Windows, unless you're using Java 8 JRE for work testing or something of that sort, I suggest you stick to the latest version of Java 7.
Just a casual observation, Java updated their version 8 on the 19th. This morning that update went out on my machine, and FML hasn't worked since. So I'm guessed there's an issue with JRE 8 rv20. I can force it to JRE 7, see if that resolves the issue. As an aside, OS is Windows 8.1. Java is 1.8.0.20 (right there in the pastebin btw). Edit: Going to JRE 7 does resolve the issue. I'm going to downgrade my JRE 8 to an earlier version as an experiment.
