While making an HTTP to HTPPS redirection trunk on an UAG farm with NLB when activating he configuration the following error could occur: « Error: Trunk "name of the trunk" cannot be activated due to the following: Invalid External IP address. Please choose a different IP. »
When we look in detail the HTTP trunk settings we see that by default the Virtual IP field located on Use integrated NLB is empty. When we provide the same VIP used by the HTTPS trunk the activation process goes fine.