Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Apple Update WWDC iOS App Hours Before Start
It is common practise for Apple to release an iOS app to give developers information about WWDC but for some reason this time around Apple has been working hard on enhancing video playback.
Apple released an update to the App last week and another update has just been pushed today just hours before the start of the conference.