I ran into such SSL-related issue with an old RabbitMQ.Client (version 3.5.7.0). Upgrading to a newer client (in my case 6.5.0.0 as I need to target .NET Framework 4.6.1) solved the issue without any code changes). The code was not specifying full Uri, but rather setting individual properties: HostName, Port, UserName, Password, VirtualHost.