Searched the web for ages for a solution, then found it on my own.
I don’t sync with google but keep everything within my device. Also I don’t want the google calendar on my device. So all advice including syncing were useless for me.
Then I exported contacts to SD-card and reimported them => to the device. This solved the problem and S-planer suddenly showed all birthdays as expected.uc browsershareitappvn