I would say 90% of the faulty assets I find have this particular problem.
Thumbnail errors are pretty common in the latest build of TS12. They are fairly easy to correct, either in Explorer or Content Creator Plus. I use CCP for these as it will sometimes register other errors as you save the config file.
To fix in CCP, start by expanding the asset tree on the left side. In the expanded tree, you can see an entry labeled "Thumbnails". Thumbnails should also have an individual extension below it with a -0 string. Some Thumbnail extensions have more than one thumbnail (& thumbnail extension string...1, 2, etc.) and so you will have to look at the error message again to determine which line is faulty. Some assets have 2 thumbnails in which one is legit and the other is non-existant.
Now that you've identified where the problem thumbnail image is located in the config, it is time to delete that section. In CCP, right-click the little extension line next to the "0". A box will come up with a small list of options; the option you want to choose is "Remove". By removing the "0" (or 1, 2, etc.) line that was causing the error, you should now be able to save the config and commit the asset. Eliminating the thumbnail image will not cause any harm to the asset, but there will be no thumbnail image in the Asset Details section of the Content Manager.
If you delete the entire Thumbnails container by accident, you can right-click the top line of the asset tree in CCP and in the large box of options choose "Add Thumbnails Container" or something like that. This will add the thumbnails section back in, but it will also add the -0 string back in below it. You will have to right click the -0 extension to remove it.
All assets must have a Thumbnails container, but that container can be completely empty.
I hope this helps. Good luck!
Tyler