Send Test email into Campaign does not working

Summary:

In production environment when the email is sent using the functionality "Sent Test Email" the email into the inbox is empty, not HTML is shown into the code.

When the test email is send from the Template area the email arrives into the inbox correctly.

I tested the functionality into the "non production" environment and the email is sent correctly also when I send this from the campaign and from the template area.

Can you please help me understand the reason of this behavior?

Version (include the version you are using, if applicable):

25B