Nokia Lumia 520 - What can I do if get an 86000C0A error when trying to sync Microsoft account on my Lumia phone?

Try the following:

  1. Go to Settings > backup > text messages and slide the Text message backup and Chat backup toggle switches Off and restart the phone.
  2. If the account sync works, then go back and re-enable the Text message and Chat backup.
  3. If the issue persists, tap Settings > date+time and slide the Set automatically toggle switch Off. Then set the time and date manually.
  4. Check that your phone Country/Region is set up correctly. Tap Settings > language+region. You must restart your phone for the changes take effect.
  5. Try alternative connection types (Wi-Fi, 4G, 3G, 2G).  
  6. Make sure that your mail inbox is not full. Move the emails from inbox to a temp folder. This is also to determine if there are corrupted emails causing synchronisation problems.
  7. Delete and re-create your email profile on the phone. If this fails, switch your phone to flight mode and reboot. At this point you should delete your email profile, then turn off the flight mode and re-create your email.