I got this solved now: here's the expression if anyone else is interested.
Days Open: ((DateDiff("d",[Date Received],Date(),2))-(Int(DateDiff("d",[Date Received],Date(),2)/7))*2)+IIf(Weekday(Date(),2)>5,5-Weekday(Date(),2),0)+IIf(Weekday([Date Received],2)=6,1,0)-IIf(Weekday([Date Received],2)>Weekday(Date(),2),2,0)