@DirtDawg (And Steve) "Search" was the first thing I did (way before posting the question). I searched for p1022 (case insensitive, any poster, all boards, all posts).
The search returned no results... and if you search on p1022 in the forum at the moment, the original thread still doesn't show up - this thread is the only result.
On the other hand, if you google [ p1022 site:www.decware.com ], the old thread does show up - so the outside world is aware of it. The only way I found the old thread was trawling manually backwards through older forum posts.
It looks to me like the SQL resultset is being truncated somewhere, or the query itself is timing out and being discarded.
Searching for cs800x with the same criteria (a string unique to the old thread, acc. to a google for [ cs800x site:www.decware.com ]) also returns no results.
Also; I've noticed a few times that when I click on a linked URL in a forum post (including the link to the old thread below) sometimes the website won't resolve it - it tries to use a relative url:
/www.decware.com/cgi-bin/yabb22/YaBB.pl?num=1160529549
instead of an absolute one:
https://www.decware.com/cgi-bin/yabb22/YaBB.pl?num=1160529549Which returns the expected 404 error page. Something's broken behind the scenes. I'm passing this on as bug report now.