Talked a friend into buying Lightroom 6 on Win8. I spent the afternoon trying to resolve this issue. When the authorization goes to Flickr everything seems to work, a new screen is opened that seems to say authorization completed (blank screen but codes and addresses are in the address bar). When he comes back to Lightroom and presses the Done button, only the Authorize rectangle is active (if authorize works, the Change Account and Remove Authorization rectangles are active). There is no account given and Not Authorized is at the top of the Flickr Account block. We also tried it with my Flickr account, same result.
He has another computer and before purchase, he asked if he could use his copy of Lightroom on both computers and the Adobe rep said he could. The rest of Lightroom works fine on both computers. He was able to authorize his Flickr account on his other computer.