In this case, Mendix gives you a pretty good idea what you need to do if you read the error. You see two important things:
* Failed to evaluate expression at ... round($Aanvrager/WerkUrenMaand * 0,7)
* Left and right hand side of binary expression should not be empty
From this, it's easy to see that $Aanvrager/WerkUrenMaand is empty. This can be either because $Aanvrager is empty or because $Aanvrager/WerkUrenMaand is empty. Why it's empty, I can't tell: that is something that you need to figure out in your application.
Did you debug the microflow? That way you can easily find out why and when $Aanvrager is empty or when $Aanvrager/WerkUrenMaand is empty?
Thanks for the reply!
I did read the error but I couldn't figure it out why it would be empty because if I want to calculate the "WerkUrenMaand" it works and I don't get this error. But when I don't want to calculate it but instead want to know the "Studie" this appears.
Thanks everyone! I found the problem, like Luc said I thought empty (null) and 0 were the same thing. So I added a change object WerkUrenMaand to 0 and now it works!