My server is set up with Let's Encrypt but instead of RSA I am using EC-384 key exchange. NZB360 refuses to connect and it doesn't give an error message except "Null". Regular TLS 1.2 ciphers are selected which worked before with Comodo so I don't believe that is an issue. Let's Encrypt certs are cross signed by DST so it's probably not a trust issue either. The only thing that comes to mind is the EC key exchange setup.