Gave this a whirl and created a new pfx cert from my LE pem files. Same issue with Plex tho after changing the key in the config and restarting the service. Here is the command I used:
sudo openssl pkcs12 -export -out plex3.p12 -certpbe AES-256-CBC -keypbe AES-256-CBC -macalg SHA256 -inkey privkey.pem -in fullchain.pem
