Hi Frederik, in general it can be difficult to edit pieces by dragging when you've got fillets and shelling applied.

For your case here though you can use construction techniques to edit it - draw in a rectangle like this:



Select the object and run Construct > Boolean > Difference and then select the rectangle as the cutting object which will make this:



Drag the end piece over:



And then select both pieces and run Construct > Boolean > Union:



- Michael