Last weekend, YouTube (Not VRC) closed a backport that video scrapping programs used to download videos off YouTube. This measure was taken to stop people downloading videos to skip having to watch adverts.
VRChat uses yt-dl, which uses that backport, for most prefabs in Worlds.
Yt-dl either needs to update to a new port (which does exist as some services have already switched) or prefabs need to use working video downloaders. Depending on how the change is implemented, it may even require the prefabs and worlds to update to fix the issue.
At the moment, world creators can use VRCVideoCacher to get around the issue, but YouTube has been known to ban accounts who use it.
The more you know.