This could be due to user filters present in the workbook with blank thumbnail. You can refer to discussion on this topic here: View and Workbook thumbnails not generated on Tableau Server
Hope this helps.
As mentioned by Nachiket the blank thumbnails are due to user filter being used in the workbook.
After you publish you can see the preview of the workbook for user filter on tableau server.
Please let me know if this helps.
I had the same problem after we upgraded to new server. My workbooks did not have filters but thumbnails were still blank. I stumbled across this fix, hope it works for you. If you are the owner of the report open the report inside the Tableau Server webpage and click Edit. Then click Save. Now the thumbnails should be visible.
Great solution! Thanks!