UCS Diagnostic: fullchain.pem: verification failed

UCS system diagnostics keeps saying that the certificates I use for IMAP/POP3/SMTP are invalid:

Ungültiges Zertifikat '/etc/certs/fullchain.pem' gefunden:
error /etc/certs/fullchain.pem: verification failed

But they are definitely not. The certificates come from Lets Encrypt. However, these are sent from another server via SSH to the UCS computer and are also kept up to date.

As said. The certificates are valid and everything works perfectly. This message in the diagnostics just annoys me mercilessly. What can I do about it?

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