Today a company reported random connectivity issues through an OpenVPN server. Together with these errors in the logs:

AEAD Decrypt error: cipher final failed

We have fixed reading the whole server configuration and finding this part:

# Uncomment this directive if multiple clients
# might connect with the same certificate/key
# files or common names.  This is recommended
# only for testing purposes.  For production use,
# each client should have its own certificate/key
# pair.
#
# IF YOU HAVE NOT GENERATED INDIVIDUAL
# CERTIFICATE/KEY PAIRS FOR EACH CLIENT,
# EACH HAVING ITS OWN UNIQUE "COMMON NAME",
# UNCOMMENT THIS LINE OUT.
;duplicate-cn

So. If you have multiple clients sharing the same certificate, you have to de-comment that line.

Remove ; before duplicate-cn then save and restart the OpenVPN server and everything should work again.

This task is published in the hope to reach search engines and save time to other people.