Hi Matthias,
I'm getting failed payments when customers use a coupon with a subscription. The Paymill error is '40301 RESPONSE_DATA_3D_AMOUNT_CURRENCY_MISMATCH: currency / amount mismatch'. It looks like the plugin is sending the order total to Paymill without the discount during the preauthorization.
I found the following in the Paymill troubleshooting information:
'The "amount_currency mismatch" error comes up if the amount_int value that is sent to the bridge does not match the amount that is being processed with the first transaction. The error only comes up when 3-D Secure is activated. You can avoid this error by using the same amount for the first transaction and bridge communication (amount_int parameter).'