Been struggling with a strange issue lately but I've narrowed things down, so I here's what I've found in case it helps others with the same problem. I don't think AE is the cause, but since AE is where you're likely to experience the problem, I wanted to document it somewhere.
Symptom: a project containing certain ProRes assets renders fine on Intel Macs, loads and displays correctly on ARM macs but when you try to render them, AE hangs at the start of the render.
The cause seems to be some ProRes files that have been created using ffmpeg. Not all files created with ffmpeg exhibit the problem. The ProRes files in question may not cause problems in any other software packages, but something about the way ffmpeg compresses them can cause issues with the M1 chips' hardware decoding, I'm guessing. We've been using ffmpeg to compress image sequences into ProRes files for years now without any problems, so this has been tricky to track down; it really does only seem to affect AE on M1 chips. Well, and a subtle thing in Finder (see below).
The problem seems to be in the stream(s) themselves rather than anything in the container or metadata; if I take a problematic ProRes file and drop it into FCPX, then export it as a new ProRes movie (using the same codec as the source file), the problem remains.
How to check: if you have a project that contains ProRes files, and you're finding AE hangs only when starting a queued render, check whether Finder on that particular machine is able to display a thumbnail for those source files. If not, you may need to recompress them before using them in AE.
These ProRes files show thumbnails correctly on an Intel mac:
... but when viewed on an Apple Silicon mac (same version of macOS etc), one of the files doesn't display a thumbnail correctly:
... and while all three files will work correctly in other apps on the Mac (FCP etc), if you try using that middle file in AE you may encounter problems.
To fix: you'll need to transcode the file. You can use Adobe Media Encoder for this, but make sure you transcode to a different ProRes type from the source file to force a transcode rather than a copy. Alternatively you can use the latest version of ffmpeg (v5+); the new prores_videotoolbox codecs not only produce working files but are also blazing fast on ARM Macs.
I've attached that particular problem file above to this post if someone wants to experiment. It'll work fine in FCP, and on AE on Intel macs, but you'll likely have issues trying to use it in AE on ARM.
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