This issue occurs when dynamic templates created within SendGrid do not render the thumbnail image when the template is created.
Troubleshooting
Send a test email within SendGrid's template editor
Look at the email and see if you can find any images not rendering correctly.
If you find an image not rendering in the email, go into the template editor, delete the image component, then re-upload the image.
Verify that the URLs for the images you're using in your template are correct and accessible. The URLs should be absolute paths starting with
http://orhttps://.Ensure that the images are in a supported format (such as JPEG, PNG, GIF) and are not too large in file size. Sometimes large image sizes can cause timeouts or other issues.
If your images are served from a domain that has firewall rules or is behind a content delivery network (CDN) with strict rules, it might restrict access to the image from SendGrid's servers.
Workaround
Delete any inactive versions that do not show the thumbnail
Recreate the template but do NOT clone the failed template.