The videos taken with my iPhone (IMG_1709.MOV) display a totally white thumbnail.
The videos taken with my Canon Camera (MVI_2109.MOV) display a good thumbnail.
If I load the video in the player, I advance an image and I click on the button “update”, the thumbnail is well defined.
I still had another problem today while creating a new project.
The waveform was not displayed. I remembered that this problem had happened to me some time ago. Dan had found the solution by deleting the db.sqlite3 file
I redid the same action and I found the waveform and more the thumbnail …
Remember to purge this file from time to time.