I've gotten this error trying to use magiclauncher to install Forge. Vanilla unmodded minecraft 1.4.5, magiclauncher 1.0.0, and forge version 1.4.5-6.4.0.396 (which is the current recommended build.) I am not running any other mods of any type, in fact, i just completely wiped my .minecraft folder from %appdata%, and reinstalled fresh, and am still getting this error
Any other info needed, just ask.
Log file
magiclauncher.cfg