What is a "Network error" that appears during syncing?

A Network error indicates that you’ve lost your internet connection during the synchronisation of a file or folder. The sync will try again after some time if the internet connection is restored. The duration of the disruption depends on how long the internet is unavailable. The longer the network is down, the longer it may take to recover later. However, it should never take more than one hour after the internet connection is restored.

Alternatively, you can pause and start the sync manually to force a recheck for connection.