If you don't repay everything you used during the month, your account stays in good standing as long as you pay at least the minimum payment by the 15th. This isn't a missed payment and there are no penalties.
💳 What changes is the cost. The interest-free period only applies when you repay a month's spending in full. If you repay part of it, the interest-free period no longer applies to that spending, and interest is charged on the credit you used from the day after each purchase — not only from the payment date.
An example. You spend €500 in August and repay €250 by 15 September:
€250 is well above the minimum payment, so nothing is overdue
interest is charged on the €500 for the days before your payment, and then on the remaining €250 until you clear it
the remaining €250 carries over to the next month
Interest is calculated daily on whatever you still owe, so the sooner you repay it, the less it costs. You can repay early, in any amount, as many times a month as you like.
Once you've cleared the balance in full, the interest-free period starts working again for your following months' spending.