I was logged in with my Graphisoft ID with Safari, but when trying to access the forum I was asked to "Complete registration by choosing a Username"... Today I was able to login normally
I suspect you use multiple GSID accounts on your computer (or at least there was another account in use recently), and when you visited the community, the other account's token was recognized, hence it looked like a new registration in the system.
In case you see something similar again, I suggest you try to sign in on the community from a different browser or incognito window.
