Sigh. Go to
here and see what was said. Shortly after my last post there, the problem was fixed (now it's broken again). Nothing was said about logging in on a different user ID (I wasn't aware that I was logging in on an ID).
Btw, changing libraries by running the Welcome Wizard still works properly. It's just awkward and takes longer. Also (and calibre has always done this), changing libraries adds a "book" called "MY Books" to the list. It doesn't hurt anything and is easily deleted, but it's unreadable and is annoying.