Haxy
There's a fix in 3.4.1 for this issue:
https://github.com/RenderHeads/UnityPlugin-AVProVideo/issues/2183
This is currently blocking a WIP update for VRCVideoCacher that switches YouTube playback over to use the SABR protocol.
The move to SABR is mainly to future-proof things, as YouTube keeps increasing its bot checks and the legacy streams are becoming less reliable.
There's also the possibility that YouTube could eventually remove the legacy streams entirely.
If that happens, the current playback method would stop working, making YouTube playback unavailable until a SABR-based solution is in place.
Updating AVPro to 3.4.1 would unblock that work, help keep YouTube playback working going forward, and also enable support for up to 8K YouTube playback once the VRCVideoCacher update is released.