The easiest way would be to build the server with extra debugging

  make DEBUG=1 DEBUG2=1

You can also alter the way netconf-subsystem-pro is launched in the sshd_config file:

  netconf-subsystem : default usage for SSH
  netconf-subsystem -f file | -filename file : set the trace file
  netconf-subsystem -t level | -trace level : set the trace level
  netconf-subsystem -p proto | -protocol proto : set the protocol

If -t=3 (trace-level=3) is used, then you will get a separate trace file for each session instead of deleting them as the session closes. You will see files in the /tmp directory, like /tmp/subsys-err.8456.log The PID number of the process will be used for the number part.

These files are used by default if no -f option is given