Some categories redirect to the home page, even with all plugins disabled?

Hello WP Community,

I am experiencing a strange problem – some of my category links trigger a 301 redirect to my home page. Most of them work normally, this problem only affects a few of them.

I disabled *all* my plugins and the problem persists. This redirect appears to be happening before my template’s archive.php is loaded so the problem can’t be in that file… no redirect code in my header.php or functions.php either. Have not yet tried switching to the twenty eleven theme because I don’t want to lose my widgets… widget layout is reset if the theme is changed right?

Using WP 3.3.1. On my dev server I am still running WP 3.3 and this redirect does not happen on that install.

Help! Any ideas? Thanks in advance.