Error when trying to set up an SSL/TLS configuration

I'm trying to configure an SSL/TSL configuration in our test environment so we can send ADT messages to an external server. I've verified connectivity/firewall to the external server.

type is set to Client, Server certificate verification is set to Require.

I have received a certificate from the external supplier and linked that in the "File containing trusted Certificate Authjority certificate" field. (I've also imported it into the cert store and tried using %OSCertificateStore).

My client certificate is in PEM format, the decrypted key was extracted using SSLOpen. Protocol enabled is TLSv1.2 (I've tried with the others as well.)

When running a test I get this error:- ERROR #988: SSL connection failed. SSL/TLS error in SSL_connect(), SSL_ERROR_SSL: protocol error, error:14094416:SSL routines:ssl3_read_bytes:sslv3 alert certificate unknown

Is anyone able to assist in deciphering this error, or helping me sort the configuration out?

Many thanks