Took me a while to realise what was causing it, and I can replicate it every time.
If I make a 7zip file with some simple txt files in it in my blackhole folder and then open it in 7zip, if I then start newsbin it will close completely without any messages.
I'm guessing because the 7z file is locked by 7zip newsbin just closes silently rather than just ignore the fact that it can't get an exclusive handle to the file.