Ok, thanks for the answer. So it's a "common" bug.

Youtube history does not update if the ad-blocker is on. If I turn it off, no problem, and it starts to work again correctly. This affects all the videos in the same way. No problems in others browsers. All the cookies are allowed, nothing is blocked for youtube.
This is a bit annoying as the ad-blocker is a reason why I wanted to turn to opera.

Does somebody has any clue?

Thank you very much.

If needed : Opera 37.0.2178.32, on Windows 7 64-bits, with a google account.