I started getting this, too. On the heels of the other problem, I was just about ready to leave here for good. On a whim/out of curiosity, I cleared my browser and tried logging in today and got the same result, checking the browser and taking me back to the sign-in. I didn't think of trying to sign in a second time so went to the messages and clicked on reply to see what would happen, and from that page it allowed me to sign in.

That explanation sounds convoluted, but it's just a pale reflection of the experience itself. So from now on I'm going to try logging in a second time.