If you pay a bill right before the due date, unfortunately it can occur that a reminder has already been issued which crosses in the mail with your payment.
Check in my Lebara under "Bills and Usage" in any case to see what bills are still unpaid in your account.
It could be that you have overlooked an old open bill and paid a new bill, resulting in a reminder notice for the old bill. If you have paid the right bill, you can just ignore the reminder notice. It takes several workdays for your payment to be posted to your account.