Dashboard error message

Since today (July 8), I have been receiving the following message on my dashboard:

Recent Message Delivery Issues 1 message has failed to deliver in the last 24 hours. Review the logs to identify and resolve any configuration issues.

antwerpen@meat-factory.be Email delivery failed: Unable to send an email: textContent is missing (code 400.... 4 uur geleden (see screenshot) What does this error mean?