Circular references!!! Now there's a thought! but I can't see any
though!! AAARRRGGGHHH!!!

What's strange also is that after the "unsuccesful" paste  that
actually worked the maths is wrong.
For example I have this

B C D E F G
3 Januari Februari
4 Aantal Per stuk Totaal Aantal Per stuk Totaal
5 7.00 2.86 20.00
(B5*C5)

I cut B5:D5 and try to paste it to Feb at E5.
I get this

B C D E F G
3 Januari Februari
4 Aantal Per stuk Totaal Aantal Per stuk Totaal
5 7.00 2.86 20.00
(B5*C5)

With B5*C5 in G5 showing 20!