After several responses over on the Adobe forum (https://forums.adobe.com/message/4562868#4562868) it looks like problems with third party codecs.
After removing codecs from my Mac/Library/QuickTime folder didn’t work, I took a look in my username/Library/QuickTime folder and removed the two files I found there… Voila, after a restart, PP and AE will now both successfully launch!
Below are the names of the files I removed.
AC3MovieImport.component
Perian.component
This was such a seemingly random issue (working one day, then nothing) I’m not feeling too confident about this not happening again. If anyone else has any insights into the cause, please post.
Kevin