The Scenario

You have dual NICs and you disable NIC1 which uses 192.168.1.1 as its gateway.  With NIC2 you enable it/connect it to another network which also has the gateway 192.168.1.1

Everything will work fine at this point.

When switching back to NIC1 even with NIC2 disabled and even unplugged, the OS basically can't pick up the new/updated ARP entry of the old device for 192.168.1.1 and perhaps thinks it is a security risk or spoof of some sorts and blocks it all.  You may think there is an issue somewhere else but the syslog shows the issue below.

Mar 13 23:16:58 mybox dbus[1698]: [system] Rejected send message, 7 matched rules; type="method_return", sender=":1.9" (uid=0 pid=2031 comm="/usr/sbin/dnsmasq --no-resolv --keep-in-foreground") interface="(unset)" member="(unset)" error name="(unset)" requested_reply="0" destination=":1.6" (uid=0 pid=1983 comm="NetworkManager ")
Mar 13 23:16:59 mybox dbus[1698]: [system] Rejected send message, 7 matched rules; type="method_return", sender=":1.9" (uid=0 pid=2031 comm="/usr/sbin/dnsmasq --no-resolv --keep-in-foreground") interface="(unset)" member="(unset)" error name="(unset)" requested_reply="0" destination=":1.6" (uid=0 pid=1983 comm="NetworkManager ")
Mar 13 23:17:01 mybox CRON[27807]: pam_unix(cron:session): session opened for user root by (uid=0)
Mar 13 23:17:01 mybox CRON[27807]: pam_unix(cron:session): session closed for user root
Mar 13 23:17:16 mybox sudo: pam_unix(sudo:auth): conversation failed
Mar 13 23:17:16 mybox sudo: pam_unix(sudo:auth): auth could not identify password for [one]
Mar 13 23:17:24 mybox smbd[21006]: pam_unix(samba:session): session closed for user nobody
Mar 13 23:17:41 mybox dbus[1698]: [system] Rejected send message, 7 matched rules; type="method_return", sender=":1.9" (uid=0 pid=2031 comm="/usr/sbin/dnsmasq --no-resolv --keep-in-foreground") interface="(unset)" member="(unset)" error name="(unset)" requested_reply="0" destination=":1.6" (uid=0 pid=1983 comm="NetworkManager ")
Mar 13 23:17:42 mybox dbus[1698]: [system] Rejected send message, 7 matched rules; type="method_return", sender=":1.9" (uid=0 pid=2031 comm="/usr/sbin/dnsmasq --no-resolv --keep-in-foreground") interface="(unset)" member="(unset)" error name="(unset)" requested_reply="0" destination=":1.6" (uid=0 pid=1983 comm="NetworkManager ")

Solution

 sudo service network-manager restart
network-manager stop/waiting
network-manager start/running, process 31647
 

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