I thought this might more likely be my old software versions than a bug, but worth mentioning that it's happening. I've been using LRT Pro 5.4 on El Capitan recently, and it's worked pretty well. I am still using Lightroom Classic CC v7.5 because my hardware won't support the later ones. I plan to move everything to a newer machine soon.

Today I rendered my longest sequence yet: 1,230 frames. I rendered it as H.264 1080p 25fps UHQ, 16:9 cropped, as I have with all the others.

It generated a broken mp4 that can't be opened. I've looked at the Info in MacOS for this file and it appears to match the others (which open fine), so I'm unsure why MacOS can't open it. I had a quick look in the folder that contains the images (I exported as 8-bit TIFF), and couldn't see anything obviously wrong.

Is there anything else I can check? I'm going to try rendering as JPGs but it would be nice to know what happened, even if JPG turns out OK.

Thanks!

First of all, you should use a decent video player like VLC and not the crappy Quicktime that comes preinstalled on macs.
If it doesn't play on that either, I'd recommend to render again. Try rendering in HQ, there is usually no need for UHQ, it will only stress the player no more. Also check the log for errors after rendering, if in doubt send it to me.

Thanks for the reply, Gunther. I already looked at the log and found no errors, so instead I used VLC as you suggested (I'd forgotten I had it installed already). I find the interface of VLC a bit quirky, but you were right though: it did play the file! I'll remember to use it in future.