Safari not playing videos

Just recently Safari v11.1 no longer plays many videos on my desktop (see specs below). I get this message:

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The video is bad or in a format that cannot be played on your browser

Error Code: MEDIA_ERR_DECODE

Technical details :

The media playback was aborted due to a corruption problem or because the media used features your browser did not support.

Session ID: 2018-04-12:28406535fd8247cf46df000b Player ID: jumpstart_video_1

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And if I try again I get:

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Could not download the video

Error Code: PLAYER_ERR_TIMEOUT

Session ID: 2018-04-12:ef2a385af6de7ed9330b371f Player ID: jumpstart_video_1

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iMac (Retina 5K, 27-inch, Late 2015
High Sierra (OSX 10.12.4 High Sierra)