Google login insists on creating username

edit

On sagenb.com, I log in for the first time with my Google account. It tells me to create a profile. Fair enough: profile created. I log out (nothing seems to work in Firefox).

I return to sagenb.com with Chrome. I click the Google icon to log in with Google. I get sent to the "Create your Sage profile" page (I don't want to create a profile. I already have one. I just want to log in to the profile I've already got with the exact same (Google) credentials I used to create it). If I enter my existing username, I get told it's already in use.

So, having already created a profile/username on sagenb.com by logging in with Google, how does one simply log back in to it again? Right now it's looking like a one-shot deal.