Now I found the problem why the NAT is not working and I think it is a Bug in the pfSense 1.2.3.
It looks like that the "NAT Port Forward" have got problems with "Aliases" type "PORTS"
I create a Alias with 2 Ports (80, 443) type "Ports(s). I use this in my Alias in my Port Forward rule.
After I try to get on the external IP over a internal Network, it doesn't work.
Now I split the Port Forward rule into to rules without a alias and use for the one rule the port 80
and for the other one 443. And now BINGO, it works, I can access a Webpage from the internal
network to the external address what is a port forward to our internal network.
Now I test this on a another pfSense installation that we have in a another location and I can
reproduce the problem on that one too.
So it looks like the version 1.2.3 have the bug with PORT ALIASES.
Hope this can help other users now.
Best,
PD