its important to use the certificate-import tool from ->start ->settings ->systemsettings ->internetoptions ->content ->certificates
direct import of the client-certificate will save the certificate in User-Certificate-Store and a manual copy from there to the Machine-Certificate-Store results in an auth-error (no certificate found for this computer) of the l2tp-client
maybe you can spare some time with this information … (could be measured in hours for me 🙄 )