Apple’s native VPN (setup via the Network system preference pane) supported DES encryption with OS X 10.5 and 10.6. DES encryption support was dropped with OS X 10.7 and 10.8. If you have a Firewall that won’t accept native VPN connections from Macs running 10.7+, but does accept native VPN connections from Macs running 10.6-, chances are the Firewall will need to be reconfigured to allow either AES-MD5, AES-SHA, 3DES-MD5, or 3DES-SHA protocols.