browser – My iMac won’t play embedded videos

In every browser I’ve tried (Chrome/Brave, Safari. Firefox), embedded videos will not play. The message is Media_Error_Unknown and a black screen which is no help to me. This only started a couple of weeks ago. My system is Mac OS Big Sur version 11.4.

On my MacAir I have no problems but am running macOS Big Sur 11.3. Could this be the problem? Anyone else had issues since upgrading to Big Sur 11.4? Thanks.