I'm not sure what is going on!
I recently installed TechnicLauncher (yesterday), and even then I was getting this same problem:
Whenever I want to install a modpack, it will say 0% for a split second then automatically cancels and reverts back to the modpack screen.
Here is a picture to show you what I mean:
Here, it has the loading bar...
And straight after that, it disappears and goes back to the modpack home screen.
Please help!