Sorry, broke the instance again when I tried to upgrade from NodeJS 4.9.1 up to 10.
Don't know what I did wrong but everything I did after that made things worse. Finally I restored the entire system from a three-hour-old backup. Now we're running again and it's time for me to go to bed.
Having backups means I don't have to know what I'm doing. :-)
Apologies once again for the inconvenience.