So, I followed the instructions as provided in the Announcements section. After I confirmed that I wanted to move my existing account to Steam, it gave me a message that an unexpected error occurred. After this, trying to log in via Steam still gives me the "create character" screen, and logging in to my normal account does not result in the "move your account to Steam" messages anymore.
Please send us a Support Request from your web account on this issue, then we can collect some information and go from there.