On my main site, I have to disable symposium everytime I want to use duplicator to create a package.
I have tested on a fresh install of latest wp with only those two plugins active and its the same problem.
The problem: When wp_symposium is active and you try to use duplicator. When you click to create a package, the popup appears but it is hidden behind a semi transparent veil that covers the screen so that you cannot click anywhere.
Solution: Disabling Symposium during package creation and then re-enabling it.