xl2tp and ipsec working internal, but not external

hi guys,

i want to get a vpn based on l2tp and ipsec (using F18 btw, but want to post it here, as it seems a kinda network prob).

i got everything working fine, so this scenario here:

--------internal lan----------|vpn server|------------badbadinternet-------
Android Phone --------> connect/route ---------> surf

and this one
--------internal lan----------|vpn server|------------internal lan-------
Samba Server <----------connect/route<---------------Android Phone

works fine. but this here wont:
--------internal lan----------|vpn server|------------badbadinternet-------
Samba Server <----------connect/route<---------------Android Phone

in each case, i get a connection (which is already great). But i would like to get it working, so i can manage my servers even outside if i am spare.

What issues can be in scenario 3? Routing? Masquerading?

Ports opened on the Firewall: 1701(l2tp), 500, 4500 (ipsec, t-nat), all forwarded directly from the external router to the firewall-server with vpn.

TIA for your help...:confused: