L2TP VPN stopped working

Hello Anthony Dunne ,

Thank you for reaching out to the community, are you facing this issue "security layer could not negotiate compatible parameters with the remote computer." on all the client machine or one particular ? Have you tried connecting with another machine ? Was the windows recently updated or some new security updates were applied on the client machine ?

Thanks for your reply. All client computers and users are seeing the same problem. Clients can also connect to other VPN endpoints, only this one shows the error.

Can you share your L2TP settings configured on the XG ?

https://ibb.co/wRPgHDF

https://ibb.co/zQVD59Y

Provided links are not accessible from our end, can you simply drag and drop/copy-paste the screenshots ?

Can you also share L2TP global settings. and On the CLI, select option 5. Device Management, then option 3. Advanced Shell. 
And share the Strongswan.log and charon.log while connecting the client !!
To check the live logs while connecting you can type the following
1.) tail -f /log/strongswan.log 
2.) tail -f /log/charon.log

To enable the debug service for the strongswan - service strongswan:debug -ds nosyn
Also take a tcpdump on the port 1701 while you can connect the client !!
#tcpdump -nei any port 1701 

Can you also share L2TP global settings. and On the CLI, select option 5. Device Management, then option 3. Advanced Shell. 
And share the Strongswan.log and charon.log while connecting the client !!
To check the live logs while connecting you can type the following
1.) tail -f /log/strongswan.log 
2.) tail -f /log/charon.log

To enable the debug service for the strongswan - service strongswan:debug -ds nosyn
Also take a tcpdump on the port 1701 while you can connect the client !!
#tcpdump -nei any port 1701 

Strongswan.log

charon.log

tcpdump:

Nothing appeared in the tcpdump while I was trying to connect. Tried 3 times

Hey Anthony Dunne ,
Thank you for the update, as we can see under the logs it says "No proposal found"  so please check the parameters like pre-shared key and try changing the policy used !!

Thanks, I managed to get it working by changing the policy.

Any idea why it would suddenly stop working with the original policy?  

it could be multiple various reasons so it depends, as per logs shared we can see that the proposal was not found and peer for any how reason was not providing a reading response...