Headphones is a wonderful piece of open source software. However, if you've configured Plex to require SSL for all connections, the Headphones Plex notifier will error when attempting to connect to Plex due to an SSL certificate mismatch.
Getting the SSL Certificate Info
Navigate directly to Plex:
https://direct-ip-address:32400/
View the SSL cert and grab the domain that the certificate is issued for:
Example: *.hash.plex.direct
Note: The hash is globally unique to your account and server.
Configure Headphones
On the Notifications page within Headphones, set the "Plex Server Host:Port" address to:
https://ip-address-with-dashes-not-dots.hash.plex.direct:32400
Note: As stated above, take the public IP address used to access your Plex server and replace the dots with dashes. Example: 1.2.3.4 would become 1-2-3-4
Conclusion
Using the above setup allows you to continue to enable SSL certificate verifications ensuring that you only connect to domains that are fully verified and secure.
Alternatively, you can set verify_ssl_cert to 0 in the config.ini however, in my testing the caused other issues that I did not feel like investigating.
Enjoy!