What is the ruling on one who swore an oath on the Quran regarding a specific event that he cannot remember at all, despite the event having occurred? And what is he advised to do in this situation?

If someone takes an oath regarding something, believing what they swore upon to be true, and then it becomes clear that it was false, this oath is considered an oath of no consequence (yameen laghw) according to the preponderant opinion, and there is no expiation due for it.