Same here. The Account button is completely gone. What happened?
I'm pretty sure it's an elaborate scheme by GOG to keep opening the front page, sooner or later one will cave in and move from looking at the Stardock games to purchasing them. :-P
After logging out, clearing everything (cookies, DOM storage, etc.), and opening the GOG forum page, the nav-bar seemed to behave as should, and I managed to log in. But instead of the Account tab, I could still see the Sign Up/ Log In buttons. Visiting the front page, then refreshing the forum page, fixed the issue. Seems to behave so far, so I suggest everyone give this a try.