Wrong Discount Calculation

price 599€, 10% Discount incl. 19% VAT. Should be total 539.10€

At checkout it shows:
599€ striked through 485.19€ 10% Discount name, 485.19€
VAT 77.47€ (is like 16%)
Total 485,19€

BIG difference. It actually deducted 19% off the price. My Shop runs in Gross Price mode so maybe that is a problem.

So my end price is always the same regardless of the applied VAT.