What is the values of the variables Streefnorm_A & Streefnorm_overige? Replace those variables with some values like 1 or 2 and try to reload, if you are able to reload then the problem is with your variables.
Also make sure that all the columns are there in the excel file. If nothing works then attach some sample of the two tables.
Streefnorm A-G is a variable that the user can set between 0-1. VHE_EPA is in a different table than HP_MAXHUUR/NETTOHUUR. That's why I joined the table with MAXHUUR and NETTOHUUR with VGKENMERKEN (the table with VHE_EPA)
I don't think the problem is in the variables since I copied this calculation from an expression. I want to load the expression as a field so to say. The expression works fine, only thing I've changed are the field names (e.g. HP_MAXHUUR instead of maximumhuur)