Hey there Mrietjezzz,
No problem! We'd like to help get those thumbnails showing in Finder for those files. You've also tried some great steps already!
Let's continue from there and continue ruling out what we can, but know that you may need to contact Apple Support for further troubleshooting:
- The first thing we recommend is a restart. Check out the section 'Restart' here: Log out, sleep, wake, restart, or shut down your Mac
- As you mentioned having a new M1 MacBook, it should've come with macOS Big Sur(11.0) installed, but the latest is macOS 11.1. You can verify which version number is installed by clicking on the Apple logo at the top left corner of the screen, then on ‘About This Mac’. Here's how to first Back up your Mac with Time Machine, and afterwards, here's How to update the software on your Mac.
- This link shows how to Start up your Mac in safe mode.
- To test for this in a new user, check out the steps under 'Add a user' here: Set up users, guests, and groups on Mac
If you need more help, we recommend reaching out to Apple Support here: Get Support
Take care!