Hi Martina,
You are using several statements that are joined together with 'or'; this would lead to a Boolean (true/false) calculation. You should instead use a nested series of if() statements to filter each option:
if(selected(${monthly_income}, 'do_2_000_00_kn'), '3', if(selected(${monthly_income}, 'od_2_001_00_3_000_00_kn'), '2', if(selected(${monthly_income}, 'od_3_001_00_3_500_00_kn'), '1', if(selected(${monthly_income}, 'vise_od_3500'), '0',''))))