Unexpected playback error

I’m using version 10.6.4 with Linux Mint 20. It was freshly installed a few days ago when I upgraded Linux Mint. It worked perfectly. Now, when I try and play some media (eg a TV series) I get an error message

" This client isn’t compatible with the media and the server isn’t sending a compatible media format"

What has changed? And, more importantly, can I do anything to fix it?