Noone I asked can remember seeing this oxalis here before (eg. my
grandma). It spread the last, let's say 10 years across all gardens and
places.

It is a US native. I am sooo sorry. If you could see all the terrible weeds
that we got from Europe, you would sympathize. ;-)

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)