Category page URLs on WPMU main blog


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 (the structure specified in Permalinks settings) it comes back with an error.

Any ideas where to start looking for a resolution?