I have an XBOX LIVE Gold subscription that ran through my account. However, since this is empty, I now have overdue fees. So I bought a 15 euro XBOX prepaid card that I've already redeemed. Now I still can't pay the fees because there's no credit as a payment option. Thank you for your help. (Does anyone still know how long you can have overdue fees)
In order to settle your outstanding payments, you need to add an alternative payment source. This can be done with either a credit card or PayPal.
In this way Microsoft ensures that there's no further payment default.
And then do I have to pay by credit card or PayPal?
The outstanding payment in any case. I don't know whether it will be possible again with credit afterwards.
I advise against paying a subscription with credit anyway for exactly this reason. If you want to live Xbox live for a limited time, you can also use prepaid cards which have a limited usage time.