When are you allowed to use fog headlamps also during the day?

Your answer is correct. According to German traffic rules, fog headlamps may be used during the day in conditions of reduced visibility caused by fog, falling snow, or heavy rain. These conditions limit your ability to see and be seen by others, so using fog lamps helps improve safety. However, ice on the windscreen does not affect visibility in the same way and therefore does not justify the use of fog lamps during the day. Always ensure your visibility is clear and only use fog lamps when necessary, as improper use can dazzle other drivers.