我正在将系统从 14.04LTS 升级到 16.04LTS，在此期间我运行了以下命令：
add-apt-repository --remove ppa:seriy-pr/network-manager-l2tp
升级后，我发现Configure VPN网络管理器中的 变灰并且L2TP不起作用。如果我点击L2TP，网络管理器图标会突然消失。我安装了 L2TP，L2TP 现在没问题了，但Configure VPN仍然是灰色的
相关日志：
~$ service network-manager status
NetworkManager.service - Network Manager
Loaded: loaded (/lib/systemd/system/NetworkManager.service; enabled; vendor preset: enabled)
Active: active (running) since mer 2017-04-19 04:15:38 CST; 11min ago
Docs: man:NetworkManager(8)
Main PID: 865 (NetworkManager)
CGroup: /system.slice/NetworkManager.service
├─ 865 /usr/sbin/NetworkManager --no-daemon
├─1362 /sbin/dhclient -d -q -sf /usr/lib/NetworkManager/nm-dhcp-helper -pf /var/run/dhclient-wlan0.pid -lf /var/lib/NetworkManager/d
└─1373 /usr/sbin/dnsmasq --no-resolv --keep-in-foreground --no-hosts --bind-interfaces --pid-file=/var/run/NetworkManager/dnsmasq.pi
apr 19 04:16:15 mylaptop NetworkManager[865]: Connection terminated.
apr 19 04:16:15 mylaptop pppd[2651]: Modem hangup
apr 19 04:16:15 mylaptop pppd[2651]: Connection terminated.
apr 19 04:16:15 mylaptop NetworkManager[865]: ** Message: Terminated ppp daemon with PID 2651.
apr 19 04:16:15 mylaptop NetworkManager[865]: <error> [1492546575.2128] platform-linux: do-change-link[4]: failure changing link: failure 19 (N
apr 19 04:16:15 mylaptop NetworkManager[865]: <warn> [1492546575.2128] device (ppp0): failed to disable userspace IPv6LL address handling
apr 19 04:16:15 mylaptop NetworkManager[865]: <info> [1492546575.2149] devices removed (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
apr 19 04:16:15 mylaptop NetworkManager[865]: <info> [1492546575.2150] vpn-connection[0x11b6210,1037a4fd-8d22-430a-a8d5-8f31c5bdd70b,"fast235"
apr 19 04:16:15 mylaptop NetworkManager[865]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 1 / phase 'dead'
apr 19 04:16:15 mylaptop NetworkManager[865]: ** Message: nm-pptp-ppp-plugin: (nm_exit_notify): cleaning up