I think I'm having a related problem, and I agree I have never had these kinds of problems in other community or comment sections of a site, say like the Washington Post. I created a user id as prompted when I wanted to respond to a question, but a day or two later when I come back when it prompts me for a user id I put in the user ID I used before and it says it's taken. I tried a bunch of things, not sure of the sequence now - logging and logging out of QB, clearing cookies. I think what finally worked into an incognito window and logging in. 

This community page isn't handling what's in cache or the cookies well at all, and should give more helpful error messages, like telling you to do the steps noted here. It does feel like Intuit is using old technology.