私のCentOS 6.4 x64システムでは、PPTP VPNとOpenVPNが接続に失敗し、両方ともVPN Secretのエラーを返します！ WindowsとUbuntuで設定を問題なくテストしました。また、OpenVPNの詳細設定にUbuntuで利用可能なCentOSの「プロキシ」タブがないことがわかりました！ [プロキシ]タブでもいくつかの設定を指定する必要があるため、言及します。
問題のシステムログは次のとおりです。
PPTP：
Sep 25 13:07:00 Athlon NetworkManager[1848]: <info> Starting VPN service 'org.freedesktop.NetworkManager.pptp'...
Sep 25 13:07:00 Athlon NetworkManager[1848]: <info> VPN service 'org.freedesktop.NetworkManager.pptp' started (org.freedesktop.NetworkManager.pptp), PID 3180
Sep 25 13:07:00 Athlon NetworkManager[1848]: <info> VPN service 'org.freedesktop.NetworkManager.pptp' appeared, activating connections
Sep 25 13:07:00 Athlon NetworkManager[1848]: <info> VPN plugin state changed: 1
Sep 25 13:07:00 Athlon NetworkManager[1848]: <info> VPN connection 'PPTP' (ConnectInteractive) reply received.
Sep 25 13:07:00 Athlon NetworkManager[1848]: <info> VPN plugin state changed: 3
Sep 25 13:07:00 Athlon NetworkManager[1848]: <info> VPN connection 'PPTP' (Connect) reply received.
Sep 25 13:07:00 Athlon NetworkManager[1848]: <warn> VPN connection 'PPTP' failed to connect: 'No VPN secrets!'.
Sep 25 13:07:00 Athlon NetworkManager[1848]: <warn> error disconnecting VPN: Could not process the request because no VPN connection was active.
Sep 25 13:07:00 Athlon NetworkManager[1848]: <info> Policy set 'Auto FFR' (wlan0) as default for IPv4 routing and DNS.
VPNの電源を入れます。
Sep 25 12:31:18 Athlon NetworkManager[1846]: <info> Starting VPN service 'org.freedesktop.NetworkManager.openvpn'...
Sep 25 12:31:18 Athlon NetworkManager[1846]: <info> VPN service 'org.freedesktop.NetworkManager.openvpn' started (org.freedesktop.NetworkManager.openvpn), PID 10750
Sep 25 12:31:18 Athlon NetworkManager[1846]: <info> VPN service 'org.freedesktop.NetworkManager.openvpn' appeared, activating connections
Sep 25 12:31:18 Athlon dbus: [system] Rejected send message, 1 matched rules; type="method_call", sender=":1.1" (uid=0 pid=1846 comm="NetworkManager) interface="org.freedesktop.NetworkManager.VPN.Plugin" member="NeedSecrets" error name="(unset)" requested_reply=0 destination="org.freedesktop.NetworkManager.openvpn" (uid=0 pid=10750 comm="/usr/libexec/nm-openvpn-service))
Sep 25 12:31:18 Athlon NetworkManager[1846]: <error> [1411635678.904108] [nm-vpn-connection.c:952] connection_need_secrets_cb(): NeedSecrets failed: dbus-glib-error-quark Rejected send message, 1 matched rules; type="method_call", sender=":1.1" (uid=0 pid=1846 comm="NetworkManager) interface="org.freedesktop.NetworkManager.VPN.Plugin" member="NeedSecrets" error name="(unset)" requested_reply=0 destination="org.freedesktop.NetworkManager.openvpn" (uid=0 pid=10750 comm="/usr/libexec/nm-openvpn-service))
Sep 25 12:31:18 Athlon dbus: [system] Rejected send message, 1 matched rules; type="method_call", sender=":1.1" (uid=0 pid=1846 comm="NetworkManager) interface="org.freedesktop.NetworkManager.VPN.Plugin" member="Disconnect" error name="(unset)" requested_reply=0 destination="org.freedesktop.NetworkManager.openvpn" (uid=0 pid=10750 comm="/usr/libexec/nm-openvpn-service))
Sep 25 12:31:18 Athlon NetworkManager[1846]: <warn> error disconnecting VPN: Rejected send message, 1 matched rules; type="method_call", sender=":1.1" (uid=0 pid=1846 comm="NetworkManager) interface="org.freedesktop.NetworkManager.VPN.Plugin" member="Disconnect" error name="(unset)" requested_reply=0 destination="org.freedesktop.NetworkManager.openvpn" (uid=0 pid=10750 comm="/usr/libexec/nm-openvpn-service))
Sep 25 12:31:18 Athlon NetworkManager[1846]: <info> Policy set 'Auto FFR' (wlan0) as default for IPv4 routing and DNS.
この問題を検索すると、NetworkManagerに関連するバグがあることがわかりましたが、問題を解決できませんでした。