TL;DR at the bottom - otherwise, context below
Just had this same problem, managed to fix it on my end (using a Windows machine over Wi-Fi exclusively, not sure how applicable this could be for others, but might be something similar)
The only troubleshooting steps I took before this was just to clear all the cache types in VRC from the menu in-game, and restarting the game - nothing else
Since it seems like VRChat looks at your network when loading videos for one reason or another (probably to detect misuse of some kind), resetting the network catalog worked for me
netsh winsock reset is the command - Microsoft Learn has a page on winsockif you’d like more details
Short version (from what I can tell from this and other websites): removes all the Layered Service Providers (LSPs) to give the catalog a fresh start, and LSPs should be reinstalled as needed as far as I’m aware - please note: this can break functionality in some programs so you may need to reconfigure them - please proceed with caution, don’t indiscriminately run commands (haven’t had problems myself just yet, but don’t take just my word for it)
I just ran it in Command Prompt as an administrator and restarted my PC, video players started working for me again! Albeit, not every video I tried worked, but the majority of them did, which when nothing worked before, is a success for me
Although this is when I have put in the link myself - I have yet to see if this works when others have put in links
TL;DR: Ran netsh winsock reset in the Command Prompt as administrator, then restarted my PC - most videos seemed to work after that when none did before
Might not work for everyone but could be useful - and please double check to make sure it won’t impact you in any other undesired ways