I think that deleting any bookmark you have and setting that up again from a new search is the answer. I had cleared my cache but the problem persisted.
This morning I was presented with the sign in page. I closed that tab, Googled the forum, loaded it from there and it came up with me already signed in. So I deleted my old bookmark, bookmarked it afresh from my signed in page and for now, all is fine.
We have the same problem with one of the pieces of software that we develop and support where I work. Whenever we have an upgrade we get calls from people who have bookmarked a link on their desktop instead of accessing it via their Programs menu, and this is exactly what we have to get them to do.