Error: BLZBNTAGT00000840

Can’t seem to update Wow or Wow Classic.

I built a new machine for a friend, and copied over wow and wow classic to save him downloading it all again (his connections pretty bad). To test I’ve logged in with my account, and that’s fine, but the games will not update. I’ve tried the scan and repair tool. I’ve disabled UAC. It doesn’t have any AV or anything else as yet. Restarted numerous times. WTF?