@alexandros, yeah, that was it. Sadly Hugo cannot handle webp yet, so if you reference one somewhere, as soon as you try any manipulation on it, the whole site crashes.
I issued
find . -name *.webp -exec mogrify -format jpg {} \;
find . -name *.webp -exec rm {} \;
in my content folder and used my text editor to search and replace for “webp” to “jpg”.
Now it works.