Guys, i had the same problem. It was driving me nuts. The solution is quite simple:
There is a diffrence between your line6.com username and your line6.com Email Adress.
You have to sign in with your username and NOT with your mailadress.
To see your username, check you "personal info" on the line6.com page:
https://line6.com/account/personalinfo/
Hope that helps.