Ok, Featherpad tries to open the files by URL scheme. When opening a local file, it works, but not when non-UNIX path names are included. What works for me, is prepending file:, e.g. file:c:\config.sys.
@Dave: The help and version are not shown, because the application is marked as WINDOWAPI, preventing it from displaying something on the console. I created an issue for this in my Github repo.