I recently tried to import some old Lightroom 3 catalogs into my current archive and found that I kept getting the same error message ‘Lightroom could not import this catalog because of an unknown error’ After attempting to try and fix the problem, I found this effective solution on Joe Reifers blog. Read more here Lightroom could not import this catalog because of an unknown error

Essentially before importing a catalog, any offline or missing images must either be removed or brought back online in the catalog you wish to import. Offline or missing images can be found easily by using – Libray > Find Missing Images. Once the missing images are removed or brought back online. The old catalog can be imported into a new or alternative catalog.