Issue with preview text of emails in Outlook

[MMXII]

I have an issue with preview text of emails sent by the Invision Community in Outlook (2019). When I receive an auto-generated email (e.g. a notice about an item that needs moderation), the preview line always says:

<https://www.example.com/applications/core/interface/email/spacer.png>

Here is a screenshot:

What do I have to change in order to show actual preview text here?
Is this an issue with IPS or a misconfiguration of Outlook? I do not have this issue with other emails though...