It seems that an old installation of Cisco Anyconnect prevents the OS X built-in L2TP/IPSec client from running (I posted a question here: OS X 10.7, can't log into any L2TP VPN server, but didn't get any results).
What is the best way to figure out if anything is taking over the sockets or ports required for L2TP/Ipsec? I tried some basic netstat and lsof and couldn't see anything relevant.