First thing. Ensure you are using the "
Enhanced Edition", Ravenloft: Prisoners of the Mist is not compatible with the "Diamond" or any other variant versions of NWN, only Enhanced Edition will work.
You are correct, the Ravenloft Launcher is not used anymore, do not attempt to use it.
Secondly, the error you are receiving may very likely be because your 'Player Name' has not been selected as the "Save as Default" this is a known issue with the Enhanced Edition and servers 'refusing' playernames. (see screenshot below)
Simply select that checkbox (you can undo this later) and then it will likely refuse your name one last time, the next time you login after that last failed attempt, the server should accept your login attempt.