Hello,
the error message is something along the lines of: EXPKEYSIG B188E2B695BD4743 DEB.SURY.ORG Automatic Signing Key <deb@sury.org>
How to fix? Pretty easy,
In my case it appeared in the logs for two files:postfix/postfix-script: warning: not set-gid or not owner+group+world executable: /usr/sbin/postqueuepostfix/postfix-script: warning: