SOLVED ..... it was a matter of deactivating tax in the coupon definition window
I tried to create a simple coupon, nothing fancy just 15% on whatever customers order. I used the default tax category that is set to 21%.
All works fine if i dont use the coupon, then it shows the correct tax, shipping cost etc., but when using the couponcode then i get weird amounts for the coupon and (probably connected to that) the tax.
I have a product that is priced including tax for € 14,95.
The coupon is supposed to be for 15%, so i expected a discount of €3,14 instead it shows €2,71 ?!?!
Please can someone help me understand how this works or what i do wrong.