HEY! POGRESS! I moved the charger and battery to my workshop and left it there for several hours. Tried it, it worked again! Why the difference, I don't know. The voltage (125) is the same at both plugs. It didn't work for long, but at least I know where the problem is and I can do some more sleuthing.

I feel stupid for not having tried this before, but the charge light was indicating a full charge and checking the battery in the menu always showed it as full or very nearly so. You'd think the indicators would work better than that...

Thanks for everybody's help and specially to Damon DG who pointed to the power as the probable issue.

It's so great to have hugely experienced people so friendly and available!

Roman Kroitor