Where are network manager logs? (16.04)

journalctl -u NetworkManager

That does the trick

You can also try this one. You'll see some Network Manager state changes with this one.

sudo journalctl -fu NetworkManager

e.g.,

Mar 15 09:06:37 macbookair NetworkManager[1098]: <info>  [xxx] keyfile: add connection in-memory (xxx,"tun0")
Mar 15 09:06:37 macbookair NetworkManager[1098]: <info>  [xxx] device (tun0): state change: unavailable -> disconnected (reason 'connection-assumed', sys-iface-state: 'external')
Mar 15 09:06:37 macbookair NetworkManager[1098]: <info>  [xxx] device (tun0): Activation: starting connection 'tun0' (xxx)