Idea:
Downgrade Quicktime
Long explanation:
I have a PC and a Mac, and sometimes when Quicktime would update itself for whatever reason I couldn’t work with Quicktime files in After Effects or export them. If nothing else changed on your computer, it’s likely an auto-update like I had to deal with. I spent hours installing ghetto ass codecs trying to figure out what was up, just setting it back did the trick. This was on a PC, though
Alex Harris
Videographer
https://azcorporatevideo.com