I got an unhelpful error message when setting Flickr publish service. However, I tried again and then it worked later. I just wanted to post this anyway because I think it'd be more helpful it the error message is more user-friendly. Thank you!
Such intermittent errors are caused by Flickr flakiness ("Flakr") and there's not much that Lightroom can do to handle these Flickr bugs. Flickr has been getting less and less reliable over the years as Yahoo went down the drain and got bought by Verizon.