At a guess I'd say that's what happens when you click 'more' on a page that's too old for 'more' to mean anything in that context.
Making the developer guess as to context is my least favorite way to file a bug report.
I run into that quite a bit. It generally happens when you sit on a page for a long time - somewhere between half an hour and an hour, I'd guess.
How should this to be fixed (if it does)?
5 second timer on the 'Unknown/Expired Link' page that redirects to the front page?
If you can reasonably predict when the list will expire, removing the "more" link from the DOM (replacing it with a notification to go back to the first page) would remove some of the inconvenience. There are probably a few better ways to deal with it, I just don't understand why the problem needs to exist to begin with. The same problem exists on Reddit incidentally.
pretty sure this is to slow/stop crawlers.
No, it's because the "More" links are implemented with closures, which then get garbage collected after a while.