Half solved. This post gave me the clue
https://forums.modx.com/index.php?topic=62174.0 and then I set the right path for the "phpthumb document root" key in system settings. So now a thumb is displayed for image TVs.
However, in the cliche plugin and the gallery plugin, I still have this problem. Cliche generates a broken url for thumbs
http://46.20.118.12/~tessfoll/assets/components/cliche/connector.php?action=web/phpthumb&src=%2F%7Etessfoll%2Fassets%2Fcomponents%2Fcliche%2Falbums%2F1%2F13.jpg&h=90&w=100&zc=1
however if I change it to (remove the %2F%7Etessfoll from the start of the src) it works:
http://46.20.118.12/~tessfoll/assets/components/cliche/connector.php?action=web/phpthumb&src=%2Fassets%2Fcomponents%2Fcliche%2Falbums%2F1%2F13.jpg&h=90&w=100&zc=1
I still don't know where I can change the path setting...
[ed. note: Hat-rack last edited this post 15 years ago.]