I have set up L2TP VPNs on tens of PCs without a problem.
However, I now have a PC which has the correct certificates installed but just refuses to connect. The server never registers a connection attempt from the client PC (Windows 7).
This PC is not under my control and seems to have a plethora of 3rd party utilities on it. The user is also someone who likes to meddle with the settings of their PC. I suspect that either the network ports are blocked by some other software or there is a config setting which is blocking L2TP (note that Point To Point Tunnelling Protocol does work).
Any help with diagnostics/troubleshooting would be appreciated.