The secret wasn't in the body - but it is...

I've configured the mail importer in my paperless-docker setup on my synology. In the log, In 90% of the time I keep getting "The secret wasn't in the body" and the consumer won't import the mail. But the secret is mentioned in the body of the mail.

I've tried:

• only the secret in the body
• the secret as the first word in the body

Any ideas why I keep getting the error?