Same problem here, it's a mess up of the FTB launcher team, they had to re-write a new launcher for that missing file... I think they are trying to prevent people like us from getting the new packs with cracked launchers.
Patience and i'm fairly sure someone here is working to crack the new launcher. Until then, just enjoy the mod packs that you do have for the moment. (Believe me, I've been waiting just as long for a better minecraft FTB pack.)
It's always only a matter of time, just keep coming back here and check for a new launcher. Thanks for the members that do get this new launcher cracked and working for us poor (or pir8) peoples.
EDIT: I think I found a new cracked launcher.... "greg0ree-f t b . b l o g s p o t . com/#!" Copy everything between the two quotation marks and paste into your browsers window and see if this works for you! Don't forget to remove the added spaces.
EDIT 2: Nope, the other 1.2.8 launcher won't work either, we will have to wait until a new cracked 1.2.9 launcher surfaces.
EDIT 3: Dude JUST updated the launcher, am in a newly created Horizons server world as I type.