I have the same issue. It always stops within the first 4 minutes (depending on video length).
However for me its not random. It happens once for the first playback on each video site for each browser sesssion. Meaning if I go to Youtube and play any video, it will auto stop within the first 4 minutes. When I click play the video and any other video played on Youytube in that browser session will not stop until I close the browser and start a brand new session. Then it will happen again on the first video played in that new broswer session. If I go to other streaming providers the same will happen there. For example if I go to Youtube, get the issue and click play, then go to Dailymotion and play a video the first video on Dailymotion will also stop. And the same happens if I go to any of the other video host providers that any site may use. So its related to video playback and not site spesific.
This started to happen for me around end of March and I believe there were an update to Opera at the time however I dont have proof that the update is the cause. Might just be a coincident. Other browsers does not have this issue. I have also done all the steps that @B8X is listing to ensure it is Opera and not anything else. I have also tried to enable/disable lots of stuff in settings to see if anything helps.
I have also tested on two different computers and the same happens to both with the same behavior. Running Win 10 on both, however one is Pro and the other is Home edition.
I think its related to your Windows client because I do not get this when playing videos on my phone or tablet (both android) using Opera.
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/99.0.4844.84 Safari/537.36 OPR/85.0.4341.75