We cannot replicate the issue in these terms. I mean that if we
  • use a Server with version 6.0
  • configure a Proxy Adapter with version 1.7
  • include the
    <param name="init_remote">false</param>
    line in the Adapter configuration
  • use a remote adapter based on .NET Adapter library version 1.7,
    which used to work with Server version 5.1.2

then it all works and the clients connect successfully.

So, there must be something specific in your setup.
In order to collect more information, the first step is to identify which is the executable that runs the Remote Server, so that we can properly configure its log.
Can you manage to access the remote side and see how the Remote Server is launched?
Is it an executable that was compiled on your side?

If you prefer, you may rather produce a minimal subset of your deployment which triggers the problem, then pack it and attach it for us to check.

By the way, please ensure that the <metadata_adapter_initialised_first> element is set to N, as we may have been unclear on this point.