I too have been having this issue. Some specific points to my setup:
-official plex docker on unraid
-custom domain to access plex
I was able to fix this by modifying the script made for synology here for unraid to create a p12 certificate from the certificates created by the swag container. So if you’re having this issue and also accessing plex through your custom domain, set a p12 certificate up in the server network settings.
