The PPTP connection in question used to work on Vivid, now it does not connect after an upgrade to Wily.

The relevant piece of syslog follows:

Oct 29 14:03:45 monkey-mafia NetworkManager[876]: <info>  Starting VPN service 'pptp'...
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: <info>  VPN service 'pptp' started (org.freedesktop.NetworkManager.pptp), PID 7190
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: <info>  VPN service 'pptp' appeared; activating connections
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: <info>  VPN connection 'Office PPTP' (ConnectInteractive) reply received.
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: <info>  VPN plugin state changed: starting (3)
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: ** Message: pppd started with pid 7195
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: <info>  VPN connection 'Office PPTP' (Connect) reply received.
Oct 29 14:03:45 monkey-mafia pppd[7195]: Plugin /usr/lib/pppd/2.4.6/nm-pptp-pppd-plugin.so loaded.
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: Plugin /usr/lib/pppd/2.4.6/nm-pptp-pppd-plugin.so loaded.
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (plugin_init): initializing
Oct 29 14:03:45 monkey-mafia pppd[7195]: pppd 2.4.6 started by root, uid 0
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 3 / phase 'serial connection'
Oct 29 14:03:45 monkey-mafia pppd[7195]: Using interface ppp0
Oct 29 14:03:45 monkey-mafia pppd[7195]: Connect: ppp0 <--> /dev/pts/12
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: nm_device_get_device_type: assertion 'NM_IS_DEVICE (self)' failed
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: <info>  (ppp0): new Generic device (carrier: UNKNOWN, driver: 'unknown', ifindex: 8)
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: Using interface ppp0
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: Connect: ppp0 <--> /dev/pts/12
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 5 / phase 'establish'
Oct 29 14:03:45 monkey-mafia pptp[7199]: nm-pptp-service-7190 log[main:pptp.c:350]: The synchronous pptp option is NOT activated
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: <info>  devices added (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
Oct 29 14:03:45 monkey-mafia NetworkManager[876]: <info>  device added (path: /sys/devices/virtual/net/ppp0, iface: ppp0): no ifupdown configuration found.
Oct 29 14:03:46 monkey-mafia pptp[7217]: nm-pptp-service-7190 log[ctrlp_rep:pptp_ctrl.c:259]: Sent control packet type is 1 'Start-Control-Connection-Request'
Oct 29 14:03:46 monkey-mafia pptp[7217]: nm-pptp-service-7190 log[ctrlp_disp:pptp_ctrl.c:781]: Received Start Control Connection Reply
Oct 29 14:03:46 monkey-mafia pptp[7217]: nm-pptp-service-7190 log[ctrlp_disp:pptp_ctrl.c:815]: Client connection established.
Oct 29 14:03:47 monkey-mafia pptp[7217]: nm-pptp-service-7190 log[ctrlp_rep:pptp_ctrl.c:259]: Sent control packet type is 7 'Outgoing-Call-Request'
Oct 29 14:03:47 monkey-mafia pptp[7217]: nm-pptp-service-7190 log[ctrlp_disp:pptp_ctrl.c:900]: Received Outgoing Call Reply.
Oct 29 14:03:47 monkey-mafia pptp[7217]: nm-pptp-service-7190 log[ctrlp_disp:pptp_ctrl.c:939]: Outgoing call established (call ID 43272, peer's call ID 251).
Oct 29 14:03:47 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 6 / phase 'authenticate'
Oct 29 14:03:47 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (get_credentials): passwd-hook, requesting credentials...
Oct 29 14:03:47 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (get_credentials): got credentials from NetworkManager-pptp
Oct 29 14:03:47 monkey-mafia pppd[7195]: CHAP authentication succeeded
Oct 29 14:03:47 monkey-mafia NetworkManager[876]: CHAP authentication succeeded
Oct 29 14:03:47 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 8 / phase 'network'
Oct 29 14:03:47 monkey-mafia pppd[7195]: MPPE 128-bit stateless compression enabled
Oct 29 14:03:47 monkey-mafia NetworkManager[876]: MPPE 128-bit stateless compression enabled
Oct 29 14:03:48 monkey-mafia pppd[7195]: local  IP address 192.168.17.35
Oct 29 14:03:48 monkey-mafia pppd[7195]: remote IP address 192.168.17.1
Oct 29 14:03:48 monkey-mafia pppd[7195]: primary   DNS address 192.168.17.1
Oct 29 14:03:48 monkey-mafia pppd[7195]: secondary DNS address 8.8.4.4
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>  VPN connection 'Office PPTP' (IP4 Config Get) reply received from old-style plugin.
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>  VPN Gateway: 217.13.222.44
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>  Tunnel Device: ppp0
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>  IPv4 configuration:
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>    Internal Address: 192.168.17.35
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>    Internal Prefix: 32
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>    Internal Point-to-Point Address: 192.168.17.1
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>    Maximum Segment Size (MSS): 0
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>    Static Route: 172.16.16.0/20   Next Hop: 192.168.17.1
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>    Static Route: 192.168.17.0/24   Next Hop: 192.168.17.1
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>    Static Route: 172.16.32.0/22   Next Hop: 192.168.17.1
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>    Forbid Default Route: yes
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>    Internal DNS: 192.168.17.1
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>    Internal DNS: 8.8.4.4
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>    DNS Domain: '(none)'
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>  No IPv6 configuration
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>  VPN plugin state changed: started (4)
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <warn>  platform-linux: do-add-ip4-route: failure adding ip4-route '8: 172.16.16.0/20 1': Unspecific failure (1)
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <warn>  platform-linux: do-add-ip4-route: failure adding ip4-route '8: 172.16.32.0/22 1': Unspecific failure (1)
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <warn>  platform-linux: do-add-ip4-route: failure adding ip4-route '8: 192.168.17.0/24 1': Unspecific failure (1)
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <warn>  VPN connection 'Office PPTP' did not receive valid IP config information.
Oct 29 14:03:48 monkey-mafia pppd[7195]: Terminating on signal 15
Oct 29 14:03:48 monkey-mafia pppd[7195]: Connect time 0.0 minutes.
Oct 29 14:03:48 monkey-mafia pppd[7195]: Sent 124 bytes, received 94 bytes.
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: local  IP address 192.168.17.35
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: remote IP address 192.168.17.1
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: primary   DNS address 192.168.17.1
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: secondary DNS address 8.8.4.4
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 9 / phase 'running'
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_ip_up): ip-up event
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_ip_up): sending Ip4Config to NetworkManager-pptp...
Oct 29 14:03:48 monkey-mafia pppd[7195]: MPPE disabled
Oct 29 14:03:48 monkey-mafia pppd[7195]: Child process /usr/sbin/pptp 217.13.222.44 --nolaunchpppd --loglevel 0 --logstring nm-pptp-service-7190 (pid 7197) terminated with signal 15
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** Message: PPTP service (IP Config Get) reply received.
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** Message: Terminated ppp daemon with PID 7195.
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: Terminating on signal 15
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 10 / phase 'terminate'
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 8 / phase 'network'
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: Connect time 0.0 minutes.
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: Sent 124 bytes, received 94 bytes.
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: MPPE disabled
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 10 / phase 'terminate'
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 5 / phase 'establish'
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 5 / phase 'establish'
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: Child process /usr/sbin/pptp 217.13.222.44 --nolaunchpppd --loglevel 0 --logstring nm-pptp-service-7190 (pid 7197) terminated with signal 15
Oct 29 14:03:48 monkey-mafia pptp[7217]: nm-pptp-service-7190 log[pptp_read_some:pptp_ctrl.c:586]: read returned zero, peer has closed
Oct 29 14:03:48 monkey-mafia pptp[7217]: nm-pptp-service-7190 log[callmgr_main:pptp_callmgr.c:269]: Closing connection (shutdown)
Oct 29 14:03:48 monkey-mafia pptp[7217]: nm-pptp-service-7190 log[ctrlp_rep:pptp_ctrl.c:259]: Sent control packet type is 12 'Call-Clear-Request'
Oct 29 14:03:48 monkey-mafia pptp[7217]: nm-pptp-service-7190 log[pptp_read_some:pptp_ctrl.c:586]: read returned zero, peer has closed
Oct 29 14:03:48 monkey-mafia pptp[7217]: nm-pptp-service-7190 log[call_callback:pptp_callmgr.c:84]: Closing connection (call state)
Oct 29 14:03:48 monkey-mafia pppd[7195]: Connection terminated.
Oct 29 14:03:48 monkey-mafia avahi-daemon[802]: Withdrawing workstation service for ppp0.
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 11 / phase 'disconnect'
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: Connection terminated.
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <info>  devices removed (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: <warn>  (ppp0): failed to disable userspace IPv6LL address handling
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_phasechange): status 1 / phase 'dead'
Oct 29 14:03:48 monkey-mafia pppd[7195]: Exit.
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** Message: nm-pptp-ppp-plugin: (nm_exit_notify): cleaning up
Oct 29 14:03:48 monkey-mafia NetworkManager[876]: ** (nm-pptp-service:7190): WARNING **: pppd exited with error code 5
Oct 29 14:04:08 monkey-mafia NetworkManager[876]: <info>  VPN service 'pptp' disappeared

I'd say that piece of IP config looks valid to me

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: network-manager-pptp 0.9.10.0-1ubuntu1
ProcVersionSignature: Ubuntu 4.2.0-16.19-generic 4.2.3
Uname: Linux 4.2.0-16-generic x86_64
ApportVersion: 2.19.1-0ubuntu4
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Oct 29 14:07:01 2015
InstallationDate: Installed on 2015-08-20 (69 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
SourcePackage: network-manager-pptp
UpgradeStatus: No upgrade log present (probably fresh install)