Nothing shows on part of scanner

This is what I saw when I logged on today. Part of the map has nothing. Links, portals and fields aren't shown. They aren't hidden, as I cannot interact with the missing portals. They are simply gone. Restarting the app fixes the issue, but this has happened rather frequently.

What causes this? What can I do to fix it other than restarting the app?

This can happen when there are network problems. Tip: Don't start the game while moving between an area with WIFI and one without. (For example don't start the game while leaving your house, but start it earlier or when you are further away from your home WIFI).

Instead of restarting the game, use the command "Force Sync" in the Device Settings screen.