Category page URLs on WPMU main blog

Hi!

I've inherited the development of a WPMU site and subsites from another developer, and I'm having trouble with links to category pages on the main blog.

Category listing pages work fine for all sub-blogs on the site, but when my main blog tries to access category page using the URL structure http://argyllcommunities.org/blog/category/category_name (the structure specified in Permalinks settings) it comes back with an error.

Any ideas where to start looking for a resolution?

Thanks
Marcus