Microsoft. requires an encrypted connection by default and validates the certificate presented by SQL Server. If the server is not configured with a valid certificate, or the machine hosting the endpoint does not trust that certificate, this can cause an exception at startup:
Microsoft.Data.SqlClient.SqlException
HResult=0x80131904
Message=A connection was successfully established with the server, but then an error occurred during the login process. (provider: SSL Provider, error: 0 - The certificate chain was issued by an authority that is not trusted.)
Requiring encryption has been the default since version 4, which changed the Encrypt default from False to True. Endpoints that connected without encryption before upgrading to version 4 or later will encounter this exception.
To fix this, update the SQL Server installation with a valid certificate and configure the machine hosting the endpoint to trust that certificate.
It is not recommended to eliminate this exception by adding Encrypt=False or TrustServerCertificate=True to the connection string. Both of these options leave the endpoint unsecure.