How to fix LT2P over IPSEC connection dying unexpectedly

Hi i'm using network manager on debian 10 to create an L2TP over IPSEC vpn connection. for a very wird reason the process gets Termninating on signal 15. i bring up the connection with:

`
nmcli con up office

` The configuration file /etc/NetworkManager/system-connections/office.nmconnection is:

`
[connection]
id=office
uuid=cf7498fb-01c9-44c1-91a9-789d6627950b
type=vpn
autoconnect=true
interface-name=--
permissions=

[vpn]
gateway=10.0.0.1
ipsec-enabled=yes
ipsec-psk=0sbWVuZWxhb3M3MTg=
mru=1400
mtu=1400
password-flags=0
ipsec-esp=3des-sha1
ipsec-ike=3des-sha1-modp1024!
#refuse-chap=yes
#refuse-mschap=yes
#refuse-pap=yes
#require-mppe=yes
user=raspberry1
service-type=org.freedesktop.NetworkManager.l2tp

[vpn-secrets]
password=...

[ipv4]
dns-search=
method=auto
never-default=true

[ipv6]
addr-gen-mode=stable-privacy
dns-search=
method=auto

` In /var/log/messages i can see the connection and also the error

`
May 19 07:25:01 rpi4-20210210 NetworkManager:   [1621409101.4139] agent-manager: req[0xffff98002ca0, :1.53/nmcli-connect/0]: agent registered
May 19 07:25:01 rpi4-20210210 NetworkManager:   [1621409101.4195] audit: op="connection-activate" uuid="cf7498fb-01c9-44c1-91a9-789d6627950b" name="office" pid=1330 uid=0 result="success"
May 19 07:25:01 rpi4-20210210 NetworkManager:   [1621409101.4277] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",0]: Started the VPN service, PID 1336
May 19 07:25:01 rpi4-20210210 NetworkManager:   [1621409101.4445] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",0]: Saw the service appear; activating connection
May 19 07:25:01 rpi4-20210210 NetworkManager:   [1621409101.4561] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",0]: VPN connection: (ConnectInteractive) reply received
May 19 07:25:01 rpi4-20210210 nm-l2tp-service: Check port 1701
May 19 07:25:04 rpi4-20210210 nm-l2tp-service: xl2tpd started with pid 1407
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.8892] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",0]: VPN plugin: state changed: starting (3)
May 19 07:25:04 rpi4-20210210 pppd: Plugin pppol2tp.so loaded.
May 19 07:25:04 rpi4-20210210 pppd: Plugin /usr/lib/pppd/2.4.7/nm-l2tp-pppd-plugin.so loaded.
May 19 07:25:04 rpi4-20210210 pppd: pppd 2.4.7 started by root, uid 0
May 19 07:25:04 rpi4-20210210 pppd: Using interface ppp0
May 19 07:25:04 rpi4-20210210 pppd: Connect: ppp0  
May 19 07:25:04 rpi4-20210210 pppd: Overriding mtu 1500 to 1400
May 19 07:25:04 rpi4-20210210 pppd: Overriding mru 1500 to mtu value 1400
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9186] manager: (ppp0): new Ppp device (/org/freedesktop/NetworkManager/Devices/9)
May 19 07:25:04 rpi4-20210210 pppd: Overriding mtu 1450 to 1400
May 19 07:25:04 rpi4-20210210 pppd: CHAP authentication succeeded
May 19 07:25:04 rpi4-20210210 pppd: local  IP address 10.1.1.10
May 19 07:25:04 rpi4-20210210 pppd: remote IP address 10.1.1.1
May 19 07:25:04 rpi4-20210210 pppd: primary   DNS address 10.1.1.1
May 19 07:25:04 rpi4-20210210 pppd: secondary DNS address 1.1.1.1
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9512] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",0]: VPN connection: (IP4 Config Get) reply received from old-style plugin
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9524] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",9:(ppp0)]: Data: VPN Gateway: 10.0.0.1
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9525] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",9:(ppp0)]: Data: Tunnel Device: "ppp0"
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9526] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",9:(ppp0)]: Data: IPv4 configuration:
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9526] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",9:(ppp0)]: Data:   Internal Address: 10.1.1.10
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9526] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",9:(ppp0)]: Data:   Internal Prefix: 32
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9527] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",9:(ppp0)]: Data:   Internal Point-to-Point Address: 10.1.1.1
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9527] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",9:(ppp0)]: Data:   Static Route: 10.1.1.1/32   Next Hop: 0.0.0.0
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9527] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",9:(ppp0)]: Data:   Internal DNS: 10.1.1.1
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9528] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",9:(ppp0)]: Data:   Internal DNS: 1.1.1.1
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9528] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",9:(ppp0)]: Data:   DNS Domain: '(none)'
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9528] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",9:(ppp0)]: Data: No IPv6 configuration
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9529] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",9:(ppp0)]: VPN plugin: state changed: started (4)
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9592] vpn-connection[0xaaaafdcd24d0,cf7498fb-01c9-44c1-91a9-789d6627950b,"office",9:(ppp0)]: VPN connection: (IP Config Get) complete
May 19 07:25:04 rpi4-20210210 NetworkManager:   [1621409104.9596] device (ppp0): state change: unmanaged -> unavailable (reason 'connection-assumed', sys-iface-state: 'external')
May 19 07:25:05 rpi4-20210210 NetworkManager:   [1621409105.0093] device (ppp0): state change: unavailable -> disconnected (reason 'none', sys-iface-state: 'external')
May 19 07:26:35 rpi4-20210210 NetworkManager:   [1621409195.9260] device (ppp0): state change: disconnected -> unmanaged (reason 'connection-assumed', sys-iface-state: 'external')
May 19 07:26:35 rpi4-20210210 pppd: Terminating on signal 15
May 19 07:26:35 rpi4-20210210 pppd: Connect time 1.6 minutes.
May 19 07:26:35 rpi4-20210210 pppd: Sent 0 bytes, received 343296 bytes.
May 19 07:26:35 rpi4-20210210 pppd: Overriding mtu 1500 to 1400
May 19 07:26:35 rpi4-20210210 pppd: Overriding mru 1500 to mtu value 1400

` while connected, it seems to be working good, however i can't diagnose the "state change: unmanaged -> unavailable (reason 'connection-assumed', sys-iface-state: 'external')" which is probably what is causing the ppp client to send the signal 15 to terminate the connection. any clues?