我的 PPTP VPN 在华硕笔记本电脑上运行 Ubuntu 16.04 的最近升级版上停止工作（我认为）。我不太经常连接 VPN，所以不确定什么时候发生的。但我知道它以前在当前设置下可以正常工作，如下所示这里有类似的帖子。
我做了一个tail -f /var/log/syslog并得到了以下结果。
Nov 24 13:29:15 Asus NetworkManager[969]: <info> [1480022955.0681] audit: op="connection-activate" uuid="c53d4096-9860-40a9-8d2e-d01b01ca0d85" name="FreeVPN (Changes)" pid=2002 uid=1000 result="success"
Nov 24 13:29:15 Asus NetworkManager[969]: <info> [1480022955.0785] vpn-connection[0x11587b0,c53d4096-9860-40a9-8d2e-d01b01ca0d85,"FreeVPN (Changes)",0]: Started the VPN service, PID 4884
Nov 24 13:29:15 Asus NetworkManager[969]: <info> [1480022955.0922] vpn-connection[0x11587b0,c53d4096-9860-40a9-8d2e-d01b01ca0d85,"FreeVPN (Changes)",0]: Saw the service appear; activating connection
Nov 24 13:29:15 Asus NetworkManager[969]: <info> [1480022955.0995] vpn-connection[0x11587b0,c53d4096-9860-40a9-8d2e-d01b01ca0d85,"FreeVPN (Changes)",0]: VPN connection: (ConnectInteractive) reply received
Nov 24 13:29:15 Asus NetworkManager[969]: ** Message: pppd started with pid 4890
Nov 24 13:29:15 Asus NetworkManager[969]: <info> [1480022955.1133] vpn-connection[0x11587b0,c53d4096-9860-40a9-8d2e-d01b01ca0d85,"FreeVPN (Changes)",0]: VPN plugin: state changed: starting (3)
Nov 24 13:29:15 Asus pppd[4890]: Plugin /usr/lib/pppd/2.4.7/nm-pptp-pppd-plugin.so loaded.
Nov 24 13:29:15 Asus NetworkManager[969]: Plugin /usr/lib/pppd/2.4.7/nm-pptp-pppd-plugin.so loaded.
Nov 24 13:29:15 Asus NetworkManager[969]: ** Message: nm-pptp-ppp-plugin: (plugin_init): initializing
Nov 24 13:29:15 Asus pppd[4890]: pppd 2.4.7 started by root, uid 0
Nov 24 13:29:15 Asus NetworkManager[969]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 3 / phase 'serial connection'
Nov 24 13:29:15 Asus pppd[4890]: Using interface ppp0
Nov 24 13:29:15 Asus pppd[4890]: Connect: ppp0 <--> /dev/pts/1
Nov 24 13:29:15 Asus NetworkManager[969]: Using interface ppp0
Nov 24 13:29:15 Asus NetworkManager[969]: Connect: ppp0 <--> /dev/pts/1
Nov 24 13:29:15 Asus NetworkManager[969]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 5 / phase 'establish'
Nov 24 13:29:15 Asus pptp[4898]: nm-pptp-service-4884 log[main:pptp.c:350]: The synchronous pptp option is NOT activated
Nov 24 13:29:15 Asus NetworkManager[969]: nm_device_get_device_type: assertion 'NM_IS_DEVICE (self)' failed
Nov 24 13:29:15 Asus NetworkManager[969]: <info> [1480022955.1297] manager: (ppp0): new Generic device (/org/freedesktop/NetworkManager/Devices/14)
Nov 24 13:29:15 Asus NetworkManager[969]: <info> [1480022955.1345] devices added (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
Nov 24 13:29:15 Asus NetworkManager[969]: <info> [1480022955.1353] device added (path: /sys/devices/virtual/net/ppp0, iface: ppp0): no ifupdown configuration found.
Nov 24 13:29:15 Asus NetworkManager[969]: nm-pptp-service-4884 warn[open_inetsock:pptp_callmgr.c:352]: connect: Connection refused
Nov 24 13:29:15 Asus NetworkManager[969]: nm-pptp-service-4884 fatal[callmgr_main:pptp_callmgr.c:134]: Could not open control connection to 195.154.69.175
Nov 24 13:29:15 Asus pptp[4899]: nm-pptp-service-4884 warn[open_inetsock:pptp_callmgr.c:352]: connect: Connection refused
Nov 24 13:29:15 Asus NetworkManager[969]: nm-pptp-service-4884 fatal[open_callmgr:pptp.c:526]: Call manager exited with error 256
Nov 24 13:29:15 Asus pptp[4899]: nm-pptp-service-4884 fatal[callmgr_main:pptp_callmgr.c:134]: Could not open control connection to 195.154.69.175
Nov 24 13:29:15 Asus NetworkManager[969]: Modem hangup
Nov 24 13:29:15 Asus NetworkManager[969]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 11 / phase 'disconnect'
Nov 24 13:29:15 Asus NetworkManager[969]: Connection terminated.
Nov 24 13:29:15 Asus pptp[4898]: nm-pptp-service-4884 fatal[open_callmgr:pptp.c:526]: Call manager exited with error 256
Nov 24 13:29:15 Asus pppd[4890]: Modem hangup
Nov 24 13:29:15 Asus pppd[4890]: Connection terminated.
Nov 24 13:29:15 Asus NetworkManager[969]: ** Message: Terminated ppp daemon with PID 4890.
Nov 24 13:29:15 Asus NetworkManager[969]: <error> [1480022955.3127] platform-linux: do-change-link[15]: failure changing link: failure 19 (No such device)
Nov 24 13:29:15 Asus NetworkManager[969]: <warn> [1480022955.3128] device (ppp0): failed to disable userspace IPv6LL address handling
Nov 24 13:29:15 Asus NetworkManager[969]: <info> [1480022955.3147] vpn-connection[0x11587b0,c53d4096-9860-40a9-8d2e-d01b01ca0d85,"FreeVPN (Changes)",0]: VPN service disappeared
Nov 24 13:29:15 Asus NetworkManager[969]: <info> [1480022955.3196] devices removed (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
Nov 24 13:29:15 Asus NetworkManager[969]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 1 / phase 'dead'
Nov 24 13:29:15 Asus NetworkManager[969]: ** Message: nm-pptp-ppp-plugin: (nm_exit_notify): cleaning up
Nov 24 13:29:15 Asus pppd[4890]: Exit.
我尝试过多个外部（托管）VPN，但出现同样的错误。我还尝试过自己的 dd-wrt PPTP VPN，并且成功了。
有人知道如何修复这个问题或者出了什么问题吗？