I'm working on a new WordPress site (www.totallysoundhealth.com.au/wp/) that is primarily a static website that has a blog as well, and the permalink settings are "Month and name". The initial blog posts are displaying fine, but clicking on any of the category names in the sidebar returns a page not found error.
I've encountered the same problem in another site that I manage, and in that case I just removed the category links from the sidebar. That site has the same permalink settings.
Both sites are hosted with the same ISP.
What am I doing wrong?