Howdy DavidC42,
Thanks for reaching out in the Apple Communities. We understand that you’re having issues when trying to play a video. We can help!
Try booting into safe mode, logging in and testing to see if the issue persists. You can find the steps for safe mode located here: How to use safe mode on your Mac
If you’re still noticing issues after safe mode, check to see if this is something that's isolated to just your user on your Mac, or if it's system-wide. Please use the steps from the section “Add a user” to create a new user on your Mac: Set up users, guests, and groups on Mac
If the issue persists, please reach out here for additional support: Get Support
Have a fantastic rest of your day!