I just set up a VPN system from my home network to my work network running Server 10.4.3. I kept running into that same problem and it wasn't until I opened up some ports on my work firewall that everything worked flawlessly. I am wireless @ home to a D-Link router, and wired @ work to a router, getting a dynamic IP from our ADSL provider, with server, not router giving DHCP. I did have to change my home network DHCP settings, 192.168.3.0 for home, and 192.168.0.0 for work. Conflicts occur if they area the same.
Here is the post I started, which helped very much. This covered everything but which ports to open. When I get to work I can post them here.
http://discussions.apple.com/message.jspa?messageID=3064810#3064810
Have fun and hope some of this helps.
Kevin
15" Powerbook 800 Mhz DVI Mac OS X (10.4.7) Wishing to upgrade...