More clues:

If I clear browser session (clear Cookies + Site Settings + Cache) I can get Una to serve me a login window - only once. I enter my credentials and it LOOKS like it's logging me in (doesn't throw any error dialog) but then I remain Unauthenticated. Then, if I try again to log in (by clicking again on the Unauthenticated User icon in upper right) the page just flashes - the tool is broken.

Here are some truths:

• I can reset browser session (as above) and the login tool will work again, as described above .
• This is behaves consistently in both Firefox and Vivaldi (chromium) browsers.
• I'm on Pop_OS! (linux) with up-to-date browser versions
• I discovered this after Vivaldi was behaving poorly (not allowing me to type certain uppercase characters or invoke keyboard copy/paste functions within browser pages). Through Vivaldi support search I found that Vivaldi preferences had become corrupted and need a line removed. This fixed my browser issues, and led me to discover the behaviors described above.

Based on the last bullet point, I suspect Vivaldi spat some garbage and corrupted the UNA DB, somehow.