Video won't play on iMac Safari running Mojave

Last night I uploaded a short video to a newly created gallery on my SmugMug site. Immediately after SmugMug had finished processing the video I was able to play it within the default Safari browser on my 2013 iMac running Mac OS Mojave (10.14.6). However, a few minutes later the video refused to play on my iMac. Upon clicking the play button I got a gray screen with play icon in the middle and a slash line across the play icon.

Switching over to Chrome browser on my iMac, the video played fine. It also plays fine via my iPad.

Just in case it was a corrupted upload, I uploaded a .MOV version (the first one was .mp4). It, too, would not play back in Safari on my iMac.

I suspect this must be some sort of codec change in how SmugMug is compressing videos. I suspect upgrading my iMac to a newer OS will fix the issue but I need to keep this particular iMac on Mojave due to frequently used older apps that won't run if I upgrade the OS.

Is there a way for me to choose the codec that SmugMug is using for videos uploaded to my site (if, in fact, that is the issue)?

This is the video in question:

https://www.thephotobooth.net/Misc-Videos/Under-the-tree/n-hqh8dG/i-NDPpzJL/A

Mark