On a Windows 7 client I was getting the following error message after the VPN connection was completed and everything looked like normal. While the connection was completed (and the “lights” were green) the user could not do anything on the net. The log message on the client was repeating:

The data area passed to a system call is too small

In such cases it may help to check whether both sides have (or have not) comp-lzo set and set at the same value. In my case this was solved by setting:

comp-lzo no

at both the server and the client(s).