This does the trick! I think for the actual report I want to add every field that I want to group by in the same way as Table1[AccountNumber] is added here, right?
The only thing that I needed to change is that I should not use [Fiscal year] from the main query, but [Jaar] from the time dimension that I joined to.
Next to that I will also look into the "ALLEXCEPT" function. I did already play around with SUM combined with CALCULATE, but didn't find this.