Translations:IPsec L2TP VPN server/151/en

This one occurs only when using PKI. It means the subjectAltName does not match the server that the client is connecting to. This often occurs when using dynamic DNS - the certificate has the internal name rather than the external name. Either add the external name to the certificate, or disable "Verify the Name and Usage attributes of the server's certificate" in the connection definition, under Security -> Networking -> IPsec.