I'm able to somewhat reproduce an issue with buffered urls, where instigator ID is detected as invalid even though the person who set the video has never left the map.
It may be related to the issue https://vrchat.canny.io/bug-reports/p/build-738-buffered-triggers-fail-to-replay-for-new-joins-if-instigator-leaves but I'm creating a seperate canny just in case it is not, as it happens without instigator leaving the instance. To my knowledge this was not present before build 735, and likely started with build 738.
Map: Bedroom Theatre
Player A joins the map.
Player A selects stream player.
Player A plays video 1.
Player B joins the map. Video 1 loads correctly.
Player A plays video 2.
Player B's client loads and plays the video as expected.
Player B rejoins the map.
Player B may or may not load the video and get the error instigator ID invalid, despite the person setting the video still being in the map.
Now the above may or may not happen to you at the second video, but at n+1 video where n > 0.
I seem to be getting the error consistently when playing the following videos rejoining on at least the 3rd video will result in these errors.
Its possible that this bug is only the result of using alts on the same computer as that does result in some odd behavior such as all clients receiving the friends list of the latest login etc.
If the bug is related to the other issue kindly close or merge this post.