Thumbnail generation requires access to the “temp” folders on your server-side disk. Please ensure that the default configured “temp” location on your server is write-accessible to the Zoom Preview server process and the related thumbnail generation tools that Zoom internally uses (such as FFMPEG, IMAGEMAGICK etc.).
Add the following line in your preview-server.conf to set up a new “temp” location to which the processes have write access to:
In general, for any thumbnail failure, you can enable additional logging by following the steps given in the article here