If the following error message is listed in the Avira Mirror Updater log file after a failed update attempt, then the problem is related to the certification.
ssl.SSLError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:777)
Please verify on the affected system whether the "COMODO RSA Certification Authoritiy" certificate is installed and up to date.
Once the certificate is refreshed, the update will work properly again.
For more information, see this external article.