Did you try trashing the prefs file? Haven't encountered that with QT, but I've had other apps do it, and it's usually just a corrupt prefs file.
"I really need a new tagline"
—P M Evans
1. Yes, I trashed the com.apple.QuickTimePlayerX.plist file.
2. When I launched QTPX, it would open one of the last .mp4 movies I created two days ago.
3. A new .plist file WOULD NOT be created in preferences.
4. After playing the .mp4 video or even just closing it > QTPX would crash into "Not Responding"
5. Force Quit
6. I copied this same .mp4 file to another external drive
7. Deleted .mp4 from the existing hard drive so that the app would not locate it at launch.
8. Launched QTPX > works fine > creates new .plist file
9. Copied the .mp4 file back to my hard drive
10. Launched QTPX > File > New recording > works fine
11. File > Open .mp4 > CRASH
12. Tested the other two .mp4 videos created the same day.
13. Only one causes QTPX to crash.
14. All .mp4 files created exporting from Screenflow
15. At least I got QTPX to stop launching the offending video(s) at launch, allowing me to use it.