Hi, we've been experiencing the same issues here. The solution is to upgrade to SoftRemote, by safenet. It appears they bought SSH OEM (makers of Sentinel). SoftRemote as a program in my opinion is not as easy to use or configure as Sentinel. However once configured it does work well and it does work with x.509 certificates + L2TP. We have also managed to get it to work it iKey 3000
I will post soon on this Board a How-To if anyone is interested.