The short of it is that HN uses closures stored server-side to maintain state in various places throughout the site, and purges old closures regularly. As usage has gone up, what counts as an "old" closure is now probably unreasonably short. Restarting HN also invalidates all existing closures.
There is little excuse for such a poor user experience here. At least give me a link back to the home page and don't be so cryptic with the message...
You would think this site would work flawlessly. Perhaps HN feels you shouldn't be on here unless you know the pitfalls. HN doesn't seem to respect the community. Would PG return to a site with such a shitty user experience? Absolutely not. I guess YC subscribes to "do as we say, not as we do..." when mentoring batches.
Personally I'm of the opposite camp, that it's nice to make your app/site as easy on the eyes and sensibilities as possible, but then HN is not exactly trying to grow or get a million members or sell anything. It's just here to foster constructive discussion of hacker-related news.