You may need to point the software at the discovery endpoint instead: opc.tcp://host:62541/discovery. This is at least what you would need to do with UaExpert via its "custom discovery" flow.
The server has two endpoints: one for discovery, one for sessions. The discovery endpoint is unsecured, and the session endpoint only allows secured connections by default. You're pointing your software at the session endpoint, and despite configuring it to use security, it's probably trying to start with an unsecured GetEndpoints request to look up the endpoint information.