I had exactly the same thing happen to me despite downloading a brand new Mojave installer. Then I realized, the problem is launching the installer off an external HDD. As soon as I moved it back onto my internal boot SSD, it launched just fine.
Does this make any sense?
Also, assuming this can’t be remedied, it indicates I cannot use this copy on an external drive as a rescue installer. Perhaps its only use to me is to make a bootable USB stick that I in an emergency would use to boot from and install. Kind of a bummer.