I had a working IPSec VPN configuration with M0n0wall (1.8.1). I upgraded to SmallWall (1.8.3) and so far I can confirm the IPSec configuration is the same as bevor the upgrade.
If I open a tunnel within my MAC (IPSecuritas 3.4) I get connected and I'm also able to send ICMP packets through the tunnel which are responded by the targeting node but the ICMP replies from the node are not reaching the IPSec client anymore for some reason.
I do have the same behaviour with my Windows 7 system which uses ShrewSoft VPN client (2.2.2).
Well the initial diff was hard because one had been running a while... But one thing that I noticed was "Aggressive" negotiation. This can be a problem when you have both on demand and permanent tunnels. It was not in m0n0wall, but in fixing it for l2tp, some of the failsafes had to be removed. Can you set everything to "Main" and see if the problem is still in there?
I configured "Main Mode" anywhere but either way it doesn't work. In "Aggressive" mode I get connected to SmallWall but returning traffic doesn't pass SmallWall somehow. I don't understand what that could be.
Can l2tp used for the same purpose as Mobile VPN or for site-to-site VPN only? For me it would be better logically to stay with Mobile VPN (cos of less change). I use Linux, Windows and OSX to access remotely my lab environment.