I have the exact same problems, and I have no idea how to fix this crap.
It worked before, but all of a sudden this doesn't work anymore today.
Yes, I have a paid Minecraft - since Alpha.
Yes, it does work in both the vanilla launcher and the minecraft webpage.
Yes, I'm logging in under my email-adress.
Running Windows 8.
I am going to try and redownload FTB and see if that does not work, if that does not work I'll need some assistance.
Okay, just tried deleting the FTB stuff, I removed and re-added the username and password and... Still nothing. I give up.