Okay, I reduced the group set to test a.b.nl and a few small ones to see what the NB log would say.
It starts with
[20:13:05] HIGH NNTPSocket - DATE
[20:13:05] HIGH NNTPSocket - 111 20120206191305 UTC
[20:13:05] HIGH NNTPSocket - GROUP alt.binaries.pictures.manga
[20:13:05] HIGH NNTPSocket - 211 134512 1888992 2023503 alt.binaries.pictures.manga
[20:13:06] HIGH NNTPSocket - GROUP alt.binaries.nl
[20:13:06] HIGH NNTPSocket - 211 1900000001 247483647 2147483647 alt.binaries.nl
[20:13:06] HIGH NNTPSocket - GROUP alt.binaries.nl
[20:13:06] HIGH NNTPSocket - 211 1900000001 247483647 2147483647 alt.binaries.nl
when I invoke 'update headers'. Sounds like the NNTP server of the ISP is stuck at 32bit maxint there in the 211 response.
I have to mention that fetching data from that group via nzb files does fine, and given that this maxint issue was visible here for about 4 weeks now (I was looking for the how and what and a way around; didn't get far, that much is obvious), my _guess_ is that there's a bug in the GROUP response code there; I have googled for this issue too, but I must be completely down on my google fu as I haven't found this XOVER issue being mentioned anywhere in public forums or elsewhere.
Before I did the last test now, I deleted everything in the newsbin data directory except the nbi files there, so all db3 files and everything have been nuked before the test. a.b.nl headers are fetched and given the 211 response there the last reaction is
[20:13:18] HIGH NNTPSocket - XOVER 2147479923-2147479933
[20:13:18] HIGH NNTPSocket - 224 data follows
[20:13:18] HIGH NNTPSocket - XOVER 2147483547-2147483557
[20:13:18] HIGH NNTPSocket - 224 data follows
[20:13:18] HIGH NNTPSocket - XOVER 2147479923-2147483647
[20:13:18] HIGH NNTPSocket - 224 data follows
and a.b.nl 'headers download' is 'stuck' in the Download List view. (Second run indicates that the server took a little while longer to arrive at that last 224 response, BTW)
Which to me seems there's something going wrong again once NB gets to the header at id maxint.
I've also run a third test with your Range.db3 delete tip; dialed down the download days to 1. No change: server fetches a few thousand headers (or so it seems to state) and then it's stuck at maxint again.
Haven't checked with other header download tools yet to see if they survive, but is sure looks like a NNTP server bug (or two).
Anyway, will also 7zip my .nbi file and PM it to you (@Quade) so you can verify. Will dial the #connections down to 2 so we don't collide due to #connections limit at 4.
Thanks for the advice and the help; it's much appreciated.