I have tested around a bit and figured out that by default VRChat will not play videos that use AV1, however when installing the AV1 Video Extension from the Microsoft Store ( https://www.microsoft.com/store/productId/9MVZQVXJBQ9V ) AV1 videos will play without issues.
Would it be possible to implement AV1 support into VRChat or at least install the Microsoft Store app by default to make AV1 work reliably for all users instead of only some?