Applying a discount code for items does not give correct invoice

I sold three items

136,50
100,00
76,00

For a Total of 312,50 €

The customer applied a fixed amount discount code for the cart of 300 €
She correctly paid with Paypal 12,50 €

The invoice shows 312,50 € subtotal, after applied discount of 300 €: Grand Total of 47,89 € excl. tax, 2 € tax and a grand total of 49,89 including tax.

Something is wrong here and I have no explanation. Can somebody give me a hint of what went wrong?

Thanks.

Martin Marsian

PS: This is Magento 237p4