I’m using Let’s Encrypt for my certificate and it was working fine when I used older encryption options (ECDH-RSA) but I just changed to the latest recommended key type and encryption options: ECDSA_P384 and Plex Media Server appears to be unable to use the certificate generated.
To reproduce, use the acme script here to generate a Let’s Encrypt certificate:
use the option --keylength ec-384
Now if you try to use the certificate, it appears that Plex silently ignores it and uses an autogenerated one.