I was having this issue previously. Did you try looking into your folder permissions where your FTB launcher is? turns out my folder had a weird security permission on it and it wasn't letting FTB update any of my modpacks and thus would give me this error!
I hate these methods, I find it super annoying when people try to pull stuff like this. Could you imagine if they did become popular this way? are these the people who deserve that kind of fame?