Been doing the exact thing for the last 3days even in different browser
I thing my issue is, I that bought the game without creating a account first, my account was created from the steps I followed when I bought the game, in these steps I entered my Email address and after payment I could see I was signed in and could see in my profile I'm marked as a BACKER.
never in the process I was asked to enter a password, I only realized this when I completed the game download and wanted to login.
Hope this makes any sense