IIRC, the way that HN is set up, it re-distributes the "fnid=NSgN9i46gR" parameters at the end of the string to the first/second/third/etc. pages.
Reddit (a similar example) does this, but it also does it in such a way that the parameters "?count=50&after=t3_ihoin" fit into a SQL(/alternative DB) select statement.
This would probably be a better way for HN to do it, but they probably have a reason for why it's currently done this way .
Their next page URL's should either be page based ("show me page 4, even if page 4 has changed") or based on the next article ("show me the page that contains article 1234"). You might see a few duplicates are miss a few articles but you wouldn't get "unknown or expired link" errors.
Edit: I remember a few years ago it would take 30 minutes to an hour before it expired. Maybe it deletes the oldest tokens first.
So, overall the server works better. When there's load, part of this include bailing sooner on your troublesome request. ;-)
In your case, this may include no longer having memory/context for stale pagination links. I deal with this by using a browser extension to load several pages of links at once. (Without a browser extension, one can achieve the same results by opening those links manually in new tabs and then working one's way through those tabs at one's leisure. I'd advise not overdoing this, though; no need to burden the server with requests you will seldom/never use.)
(I may be wrong as to the memory management. Again, this is just my vague understanding/recollection.)
The extension I was referring to in my grandparent comment is:
https://addons.mozilla.org/en-US/firefox/addon/repagination/
(Note that there is another, older "Re-Pagination" extension that was abandoned. IIRC, this one picked it up and updated it.)
PLEASE BE CAREFUL if you use this extension. If you do not choose a limited page count, it is easy to unduly load or overload a server. This include the HN server, and such request behavior will get you cut off or banned (rightly so; please work to preserve HN resources).
I use it, infrequently, a time or two a day to load the first few top pages of results, which I then work through at my leisure.
I also use a bookmark to transform all the link href values to open in new pages/tabs. That way, I don't lose the browser-generated multiple-page page (as it were) by forgetting to right click or shift-click to open a link that appears on it in a new tab.
At the time I adopted it, IIRC I read through the code enough to determine it was not malicious. I've been perhaps lax in not checking all of the updates.
I see its max version is listed as 3.6 . I'm actually still on 3.6; when I go to 5, I guess I'll see whether it still works with max version overridden.