Please be aware that OBS v24 to v26.0 does not support OBS.Ninja. Use the Electron Capture app or a different version of OBS that does support it. (v23 or maybe future versions of OBS)
Ensure that the “Enable Browser Source Hardware Acceleration” checkbox is checked in the advanced settings. If you still get just black video when it’s checked, then you can try:
Updating your graphics card drivers
Go to the windows setting for “Graphics Settings”. You may see OBS or SLOBS under programs listed for “graphics performance preference”. Make sure OBSor SLOBS is “high performance”. Turn on hardware accelerated gpu scheduling.
Try a different video codec; perhaps &codec=h264, which can be added to the view links
Check to make sure you are not behind a corporate firewall or on a VPN
Disable any Anti-virus or other security software
Uncheck the Hardware Acceleration checkbox -- does it work now? If so, it may be choppy, but perhaps still usable
You can also download the Electron Capture app, and use that instead of OBS browser source. https://github.com/steveseguin/electroncapture
As a final resort, consider using a Cloud-hosted version of OBS instead, such as on Paperspace.
If the hardware-acceleration checkbox is not checked, check it. Does it work now?