Show me the reason: Why did my network fail? (Show error messages from NetworkManager)

The method is for systemd users.

In order to show the error messages printed from your NetworkManager, you need to manually start it from your command line.

Here's the instructions:

systemctl mask NetworkManager.service
systemctl stop NetworkManager.service
NM_PPP_DEBUG=1 /usr/sbin/NetworkManager --no-daemon
systemctl unmask NetworkManager.service
systemctl restart NetworkManager.service