and Portforwarding 443 (NAT)

SG 125 UTM9


we use a transparent proxy with an authentication against LDAP. If an user enter an URL, he will redirect to and has to login.
Because SSL-certificate-problems with iOS-Devices we change this URL to our own domain and give them a public IP. Now appears, for a login. Thats fine. We cannot add the UTM-certificate, because the mobiles are private.

Now we want to use the same public IP-address and Port 443 for an internal Webserver and forward this IP/Port via NAT. 
Result: The address will forwarded to the webserver and the login page not appears.

Does anyone have an idea to make both work?
Thank you Heiko.