Thank you megavolt and “re-welcome” also
That helps a lot, indirectly. I had in fact already checked for upgrades on opening Pamac it proudly announced 100% up to date (being from the latest ISO, one would hope for that). But I had not considered running pamac commands from the terminal.
This narrows down the problem to the GUI. The problem being that the opportunity to authenticate never happens, the authentication menu never appears (as far as I can tell, not even for a brief instant, and given things are running very slowly on a HDD through VBox, it probably would be visible if it ever happened).
But at least I have a workaround, eg
$pamac build XXX will successfully call for authentication, accept the right password, and proceed to install XXX. (After I have previously run pamac as root to switch AUR on.)
I’m calling this a workaround rather than a solution, I can’t really say running a major system GUI application by using only the command line version is a solution per se.
I’ve been using Manjaro on my dedicated home desktop for a year or two now, I’m prepared to spend time looking for a solution and comfortable with the command line. But it worries me that a brand new user checking out Manjaro for the very first time, spends an hour installing the system and then apparently cannot install any application at all, with an error message that offers no clue of what is wrong, is not going to be impressed.