BuddyPress.org

Any chance you have some kind of site-wide category plugin installed? If not, it sounds like something is corrupt in your “Default Category” setting, in which case going to the admin side and changing the default posting category to something else, and then changing it back to whatever you want should reset it without you needing to dive into phpMyAdmin.

BEFORE you do that though, maybe it wouldn’t hurt to take a look at phpMyAdmin and see what “Default Category” is set to?

If it’s happening on every blog, with every post, then something is getting loaded that shouldn’t be.