I don't get this one, but sometimes when I hit the New Posts button, it says "Invalid Redirect, log in." I'm already logged in, but when I log in again, it goes into an endless cycle of "Invalid Redirect, log in".
My experience is similar. When for some reason I want to use my moderator tools I always have to sign in again, but a few days ago when that happened it started this. So, I tried to sign in a couple more times, with same result. Then I finally noticed that I was still signed in, even though it said I wasn't, so I just ignored it and it continues to work fine. It is extremely frustrating to always get that sign, so I started this thread.
Similar issue for me, when I open "new posts" link from favorites, it keeps asking me to login over and over again. If I open some forum directly, it opens without logging me in. However I just refresh the page and suddenly I'm logged in. This is the only page doing it to me. There seems to be some issue with cookies, after some timeout (an hour or so), I need to repeat this - load forum, hit refresh, load new posts. It used to work correctly, but for the last few months it's been broken (clearing Chrome cache/cookies has no impact).
Now when I log in I get a notice that I submitted a name and password that are not authorized, but the box in the upper right corner says welcome, and my name. I can then hit the "new posts" button on the menu above the page and it works normally. I am clearly signed in, or I couldn't post this. I didn't think to copy the page url, but I will try to remember next time.