Okay, I'm trying this with my test account.
After logging on with a non-plat account, I get: "Please select a membership type
An account with the same email address already exists. Please login to Your Account.
If you have not completed your order, you will be able to complete it after you login."
I click on "Your Account", and in the "Sign In" page, I enter my username and password... looks like I can't sign in with the other account because there's cookies or something else on my Samoth account. I can log in with my Samoth account.
So you might need to clear your cookies. When trying to log in on my test account, the screen "blinked", and did nothing. (The username remained, but it cleared the password. This sounds like what happened to you.)