That is normally an issue between the server and client where they do not have matching ciphers / SSL or TLS verisons. You may need to enable older or newer SSL/TLS versions in your browser. If the site is public facing the you could use https://www.ssllabs.com/ssltest/ to check the SSL/TLS versions supported and what ciphers are in use.

Thanks tjones2019, yes I did that exact check and SSL Labs simply said it could not connect. Think by default TLS1.1 is no longer supported in 15.3.1 so I wonder if certain certificates if they are old dont have capability to work with TLS1.2 or 1.3, seems odd but thats the only explanation I could think of?

Odd, not seen certificates tied to TLS versions. I wonder if duing the upgrade it lost the private key to the certificate? Not used the Serv-U software so just guessing. When you got it re-issued did you generate a new CSR from the server and send this to the supplier?

Odd, not seen certificates tied to TLS versions. I wonder if duing the upgrade it lost the private key to the certificate? Not used the Serv-U software so just guessing. When you got it re-issued did you generate a new CSR from the server and send this to the supplier?

It was from a new CSR, so maybe it's the old private key format that was not supported?