Do you have OneDrive location on another drive than "C:" (The default) ?
yes, all my one drive files are on another drive besides C:\ (which could not hold all the files anyway).
This is the main problem, and I think they have fixed it but only for the new files not the current ones!
To fix the current ones just simply:
1- Close OneDrive completely and browse to its folder.
2- Move everything that has this issue to another drive than the current one you are using for OneDrive.
"For example if OneDrive is in drive (D:) then move the files to (E:) or (C:) or any other drive"
3- When everything is moved successfully their thumbnails will work fine!
4- Now move them back to their original location again.
5- Run OneDrive and that's it !
But please make sure you completely close OneDrive first before all this process.