
I tried rebooting, tried fiddling around with Balanced, Good, Faster, Best, and Fastest coding settings and a couple others, but the last 5 attempts were all vanilla default settings for the export.

There are no special effects or anything: fresh, clean, default install of VPX, and all I did was open Video Pro X, dropped the video clips into the timeline, squished each clip end-to-end to create simple transitions and clicked Export. I am using the latest version (17.0.3.68 UDP3) of Video Pro X and there is absolutely nothing special about the video clips I'm editing: they are 4K MOVs from an iPhone 12 Mini, and all I have done so far is dropped the clips into the timeline and used the default transitions between each clip. I click "Reset" in the bottom left of the Export window).

I am using all default settings in Video Pro X, and 95% of the time I use all default settings when exporting via MPEG-4 or HEVC in 4K (i.e. The hang happens at various, apparently random times during the export, sometimes getting about 10% through the export, sometimes 50% or more. Hello, I had about a 50% hang rate on a previous video, but the video I'm working on now is hanging every single time (16 out of 16 attempts).
