Clicked category and tag links to 404 error on main site of MS install

Hello,
I have a multisite install upon which categories and tags link to a 404 error page on main site. Clicked categories and tags on other sites @ network resolve fine. I reset and updated permalink structure, checked and repaired DB but to no avail...

In the past I had the sitewide tags and post indexer plugins from WPMU installed but deleted and removed them approx 6 months ago. I have buddypress installed, but deactivated.

Any ideas on where to look for the root of the issue?

Best,
Lance