Full log of a session:

Aug 16 09:45:31 nbtxc NetworkManager[1081]: <info> Starting VPN service 'pptp'...
Aug 16 09:45:31 nbtxc NetworkManager[1081]: <info> VPN service 'pptp' started (org.freedesktop.NetworkManager.pptp), PID 5182
Aug 16 09:45:31 nbtxc NetworkManager[1081]: <info> VPN service 'pptp' appeared; activating connections
Aug 16 09:45:31 nbtxc NetworkManager[1081]: <info> VPN plugin state changed: init (1)
Aug 16 09:45:31 nbtxc NetworkManager[1081]: <info> VPN plugin state changed: starting (3)
Aug 16 09:45:32 nbtxc NetworkManager[1081]: <info> VPN connection 'customer PPTP' (Connect) reply received.
Aug 16 09:45:32 nbtxc pppd[5186]: Plugin /usr/lib/pppd/2.4.5/nm-pptp-pppd-plugin.so loaded.
Aug 16 09:45:32 nbtxc pppd[5186]: pppd 2.4.5 started by root, uid 0
Aug 16 09:45:32 nbtxc pppd[5186]: Using interface ppp0
Aug 16 09:45:32 nbtxc pppd[5186]: Connect: ppp0 <--> /dev/pts/4
Aug 16 09:45:32 nbtxc NetworkManager[1081]:    SCPlugin-Ifupdown: devices added (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
Aug 16 09:45:32 nbtxc NetworkManager[1081]:    SCPlugin-Ifupdown: device added (path: /sys/devices/virtual/net/ppp0, iface: ppp0): no ifupdown configuration found.
Aug 16 09:45:32 nbtxc pptp[5188]: nm-pptp-service-5182 log[main:pptp.c:314]: The synchronous pptp option is NOT activated
Aug 16 09:45:32 nbtxc pptp[5194]: nm-pptp-service-5182 log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 1 'Start-Control-Connection-Request'
Aug 16 09:45:32 nbtxc pptp[5194]: nm-pptp-service-5182 log[ctrlp_disp:pptp_ctrl.c:739]: Received Start Control Connection Reply
Aug 16 09:45:32 nbtxc pptp[5194]: nm-pptp-service-5182 log[ctrlp_disp:pptp_ctrl.c:773]: Client connection established.
Aug 16 09:45:33 nbtxc pptp[5194]: nm-pptp-service-5182 log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 7 'Outgoing-Call-Request'
Aug 16 09:45:33 nbtxc pptp[5194]: nm-pptp-service-5182 log[ctrlp_disp:pptp_ctrl.c:858]: Received Outgoing Call Reply.
Aug 16 09:45:33 nbtxc pptp[5194]: nm-pptp-service-5182 log[ctrlp_disp:pptp_ctrl.c:897]: Outgoing call established (call ID 0, peer's call ID 43520).
Aug 16 09:45:33 nbtxc pptp[5194]: nm-pptp-service-5182 log[ctrlp_disp:pptp_ctrl.c:950]: PPTP_SET_LINK_INFO received from peer_callid 0
Aug 16 09:45:33 nbtxc pptp[5194]: nm-pptp-service-5182 log[ctrlp_disp:pptp_ctrl.c:953]:   send_accm is 00000000, recv_accm is FFFFFFFF
Aug 16 09:45:33 nbtxc pptp[5194]: nm-pptp-service-5182 warn[ctrlp_disp:pptp_ctrl.c:956]: Non-zero Async Control Character Maps are not supported!
Aug 16 09:45:33 nbtxc pppd[5186]: MS-CHAP authentication failed: E=691 Authentication failure
Aug 16 09:45:33 nbtxc pppd[5186]: CHAP authentication failed
Aug 16 09:45:33 nbtxc pptp[5194]: nm-pptp-service-5182 log[ctrlp_disp:pptp_ctrl.c:950]: PPTP_SET_LINK_INFO received from peer_callid 0
Aug 16 09:45:33 nbtxc pptp[5194]: nm-pptp-service-5182 log[ctrlp_disp:pptp_ctrl.c:953]:   send_accm is FFFFFFFF, recv_accm is FFFFFFFF
Aug 16 09:45:33 nbtxc pptp[5194]: nm-pptp-service-5182 warn[ctrlp_disp:pptp_ctrl.c:956]: Non-zero Async Control Character Maps are not supported!
Aug 16 09:45:33 nbtxc pppd[5186]: Connection terminated.
Aug 16 09:45:33 nbtxc NetworkManager[1081]: <warn> VPN plugin failed: 1
Aug 16 09:45:33 nbtxc NetworkManager[1081]:    SCPlugin-Ifupdown: devices removed (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
Aug 16 09:45:33 nbtxc pptp[5188]: nm-pptp-service-5182 warn[decaps_hdlc:pptp_gre.c:204]: short read (-1): Input/output error
Aug 16 09:45:33 nbtxc pptp[5188]: nm-pptp-service-5182 warn[decaps_hdlc:pptp_gre.c:216]: pppd may have shutdown, see pppd log
Aug 16 09:45:33 nbtxc pptp[5194]: nm-pptp-service-5182 log[callmgr_main:pptp_callmgr.c:234]: Closing connection (unhandled)
Aug 16 09:45:33 nbtxc pptp[5194]: nm-pptp-service-5182 log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 12 'Call-Clear-Request'
Aug 16 09:45:33 nbtxc pptp[5194]: nm-pptp-service-5182 log[call_callback:pptp_callmgr.c:79]: Closing connection (call state)
Aug 16 09:45:33 nbtxc pppd[5186]: Exit.
Aug 16 09:45:33 nbtxc NetworkManager[1081]: <warn> VPN plugin failed: 1
Aug 16 09:45:33 nbtxc NetworkManager[1081]: <info> VPN plugin state changed: stopped (6)
Aug 16 09:45:33 nbtxc NetworkManager[1081]: <info> VPN plugin state change reason: 0
Aug 16 09:45:33 nbtxc NetworkManager[1081]: <warn> error disconnecting VPN: Could not process the request because no VPN connection was active.
Aug 16 09:45:33 nbtxc NetworkManager[1081]: <info> Policy set 'TopacktAP (automatisch)' (wlan0) as default for IPv4 routing and DNS.
Aug 16 09:45:38 nbtxc NetworkManager[1081]: <info> VPN service 'pptp' disappeared

when I change the Passwort I can connect. As encryption is MPPE used.