I'm having the same problems. I have tried changing download locations, changing location of the FTB executable, neither worked. Right now, I'm going to try to delete the local files except for my save, see if that works.
EDIT: And suddenly it's working. I deleted the local files and forced an update. The files downloaded much more quickly and FML started up.
I did it by deleting every folder above Saves, then forcing FTB launcher to update. I also noticed that it downloaded much faster than previous attempts, so if it's downloading very chunkily, taking a second before updating the downloaded amount, then it's probably not going to work.
That's the best I can tell, because I have no clue what I did so differently to make it work. :/
