OK, I looked at the video again, ( https://www.youtube.com/watch?v=XUMZYVKlNsA ) about 8 threads below this one, and I saw now that chippwalters did not fillet these corners as well, so he must have been aware of the problem, or by coincidence did not fillet them.
still strange though…
I know I could fillet them before extruding, but some edges did fillet without problems, and some didn't.

wim