In the case of the latter my first thought would be thermals. Different video codecs have significantly different decoding costs, and may also stress different parts of your system. You could check for that by playing that same video but not starting at the beginning and see if it's the same duration. Or jump to just before it dies and see if it plays through.

If by "downloaded" you mean The High Seas, those who provision the high seas are often on the cutting edge of using codecs with every last feature turned on to make the videos smaller to squeeze every last bit out of the encodings that they can, which can make them unusually expensive to decode. Or so I've heard.

- not always the same frame. The first three failures were within seconds of each other, possibly the same frame. I tried again the next night and it got through that part of the video, but crashed a minute later - I was able to play the video using a different app (Ubuntu's built-in Videos app from an old Ubuntu release, maybe 20.04)

This is why I built my own distro. It took a while for me to re-learn which are actually good or bad apps. I had spent so long using the garbage Ubuntu versions only to find that the same apps, when compiled from scratch on my vanilla distro, work just fine and are way more solid and reliable.

VLC was one of those. But really, you should be using MPV instead as it's a far superior video player, or Audacious (in classic WinAmp/XMMS mode) for playing audio. VLC just sucks.

Hope this helps.

Ubuntu's default player worked. I've rarely used it.

Some bug in VLC with whatever codec these use, and then some system level bug allowing that to kill the whole machine.

I had the same thought as you at first, and was so unimpressed with buggy ass VLC on Linux Mint I was going to revert to the 2.x version, when I first built my distro. It wouldn't compile however, so I was forced to go to the latest 3.x. It turned out to work fine. The problems I had on Ubuntu just vanished and it was rock solid.

Now VLC did turn out to have some actual problems in the end that shifted me to MPV permanently. But nothing like crashing the whole system. That's all Ubuntu right there.

It's also possible that something in a particular frame is triggering a bug in your driver and crashing that way. In that case, your best bet might be to transcode the video to a different codec or something.

Maybe your particular video download is from an entirely trustworthy source, but it's not unheard of that untrustworthy folks will modify a file with the intent of causing this to happen.