How are you downloading?
Although I see you say, even headers don't always download.
Also, as far as I know, a bug was introduced in RC4 about watch lists here >https://forums.newsbin.com/viewtopic.php?p=233831#p233831
I don't know if it would affect RC3 which you say has issues.
It may be a bad idea, but you could go back to:
Version 6.90, or even 6.82.
That's assuming RC3 never worked right. But with you just reporting the issue now, it must have worked at one point.
Maybe
Quade will come by sometime and think of something. He has not been as active on the forum as in the past.
One other thing you could try is changing Status Tab logging to debug to see if any more info shows.
I don't do much with NZB's via Newsbin, but I use the search and can download, and I do have some smaller groups I can get headers from, and I am using RC4.
And can download from groups.
If it's some kind of error, usually Verbose shows, but I know debug will show more info.
I guess, even if you have a bad news provider, you still would get some type of error in the log.
Newsleecher has NO support and has not had an update for several years, so not sure if that's the answer. Although I know Agent newsreader still kind of works, and they have stopped support and no updates in years. If you just do NZB's you should do a search for SABnzbd as that is very activity maintained but only downloads using NZB's.
With the "myServer None HELP 100 Help" I can only suggest deleting the news server info you have in Newsbin, any and all, and add back the server(s).
Might be some bad info, and deleting it completely and adding back may fix that.
Overall some crazy thing is happening, maybe debug may show you as again most other solutions I would suggest you should have received some error in the logs.
Because I would suggest firewall issue, but I would guess Newbin would say can't connect or connect error.
Maybe check your Antivirus, but again I think you would get some type of error would be reported if that was interfering.
It also still would not hurt to try another newsserver provider. newsgroupdirect offers good support for newsservers. You can even get a small block to test, although looking a small block is about $10 and one month full service is $9
Other than that, maybe even try contacting Newsbin support directly.
https://www.newsbin.com/contact.php