Category archive page won't show 12 or more posts?

Hi friendly WordPress people.

I have a very strange problem on a WordPress site I administer. It used to work fine, then the host had to restore the server from a backup and now it’s messed up.

On a custom category archives page, I use query_posts to display a list of old posts in a specific category (the News category). It used to work fine, displaying more than 100 posts, but now all of a sudden the page displays the header and the sidebar but no actual content (and it cuts off before the footer is displayed).

When I changed the number of posts to show to 10, it works fine. 11 old posts works fine too. But when I switch it to 12 or 20 or 50 or 100 or anything larger I get the same page with no content, just the header and the sidebar. Weird, huh?

At first I thought it could be an issue with the server’s memory or something but after further thinking I thought that couldn’t be it because there is a similar query on another page which returns a big long list of posts in a different category just fine.

I tried creating a brand new category and putting posts under it and it works until I add the 12th or so post, then it stops displaying posts. When I turn back the number of posts queried to, say, 11, it seems to work again.

I’m a bit stumped, I’ve never seen this sort of thing before. I even tried switching the site back to the default WordPress theme (“2011”) and using 2011’s default archives page. It worked fine with the default number of posts per page to display being set at 10. But when I set the number of posts to display on category archives pages to 12 using the Reading setting in the admin backend, it gave me an entirely blank page for this News category (again, other categories work fine with more than 10 posts). Very strange.

Any thoughts?