[EDIT] I marked this as fixed, but in fact it's not fixed. The login dialog works ONCE but the login is not completed. It throws no error, it just returns me to unAuthenticated user status. And after that, the login dialog cannot be invoked again. The page just flashes. So, this is not resolved. [EDIT]

FIXED. I wish I could tell you how this was resolved, but it's a mystery. I abandoned this damaged instance of Una (it was a test site) for a couple of weeks. When I returned and tried to login from the home page, this time it just worked. I have not performed any updates. Caches had previously been cleared both from Dashboard and manually. So, it's a mystery.