Hi tanjiroukun,
Thank you for using the Apple Support Communities. I understand you're running into some issues with QuickTime Player.
Does the flickering occur with every video? Do the same videos show similar behavior when played in another app? Knowing this would help to narrow things down.
When you say it fails when you try trimming a clip, what exactly happens? If you get an error, what's the exact wording?
Which macOS version are you using under Apple menu () > About This Mac?
As a test, you might try creating a temporary administrator account on the Mac to see if the same issues occur there. Here's How to test an issue in another user account on your Mac. Once you create the admin account, log in there to see how QuickTime performs and whether you receive a similar error message.
I hope this helps you in getting started. Have a great day.