This may be caused by a misconfiguration of the email template such as a template created without a subject line. To resolve this issue, a new template and message may need to be created.
  1. Navigate to Email > Templates.
  2. Edit the template in use on the message where you are seeing the issue.
  3. Select the design tab > View > Source Code > Copy the full source code.
  4. Navigate to Email > Templates > New template.
  5. Enter the required fields (name, subject like, from name, from address) and select the design tab.
  6. Select View > Source code > Paste in the code from step 3 > Save.
  7. Create a new message from the template.