Just as stated in the title.

It does manage to boot and start loading, but once it gets into the later stages of loading the mods it crashes.

http://www.mediafire.com/file/zt5cdni5adm6awq/fml-client-latest.log

The log was too long to copy in pastebin, I apologize.

You are using a (sorry but this crash happens far to often and it is realy annyoing and I am also stressed) crappy launcher. Just use the vanilla one and this crash will disappear. Also I am curious which launcher do you use ? (I thing its magic launcher or simething like this 'cause they somehow manage to load not all needed classes, so it crashes)

I'm using the old FTB launcher from before they switched to the curse launcher.

Hmm that is new... are you using a custom JVM ? And does this also happen with the vanilla launcher

Nope, no custom JVM arguments. Just tried to use a different launcher. In this case MultiMC and it booted fine with no issue while using the same modpack.

Yeah somehow the other launcher are not including all required classes so this error happens:
java.lang.ClassNotFoundException: jline.internal.InputStreamReader