Site Category archive pages generate 404

We recently replaced our root site with another via cloner.

The new theme is Modern Portfolio Pro by StudioPress.

After the replacement, the categories disappeared. In the meantime, we set up new and better categories and newly categorized a few sites. Unfortunately, however, the category archive pages all generate 404 errors. The category count for these pages on the main site category landing page indicates that the sites in the different categories are being recognized.

We’ve tried deactivating/reactivating and uninstalling/reinstalling.

Thanks in advance for your help resolving this.