Is there perhaps some security software on your controller device causing you this connection/update issue, such as VPN or Firewall software etc. If so. try disabling those things temporarily. I would also reboot your router, NAS, Sonos devices and controller device, just to give them a fresh run at attempting to update/index the local library.

If still no joy, then try moving out any ‘recent’ tracks added and give it another go… if that works then it will likely be related to the tag metadata or perhaps the format of the new tracks or the library is hitting some kind of limitation.