Hi sakuraime,
To enable an SSL certificate on a SQL Server cluster, have you specified the certificate used by SQL Server to encrypt connections in the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL.x\MSSQLServer\SuperSocketNetLib\Certificate
? This key contains a property of the certificate known as thumbprint that identifies each certificate in the server. In a clustered environment, this key will be set to Null even though the correct certificate exists in the store.
And for the client to request the SSL encryption, the client computer must trust the server certificate and the certificate must already exist on the server.
Please refer to How to enable SSL encryption for an instance of SQL Server and How to Enable SSL Certificate-Based Encryption on a SQL Server Failover Cluster which might help.
Best Regards,
Amelia