404 on all category and tag pages

I’ve got a load of different sites on my WordPress, all with a variation of the same custom theme. For each site, I copy the theme and just tweak colours and logos. The first few sites are fine, but on the last site every category and tag page resolves to a 404 error. Article/author pages and homepage are all fine.

I’ve tried searching the forums but found nothing exactly the same as this.

Any help is really appreciated!