I'm receiving this message on one of my 2015 MacBook air's. I've tried using an installed that I previously downloaded for use with @bpavlov 's upgrade os script. I know this installer worked before, but now that message comes up. Fine, whatever, I'll just download a new one from the app store. The app store installer does the exact same thing. "This copy of Install macOS High Sierra application is damaged, and can't be used to install macOS". I am running a caching server but I wouldn't think that would be the problem?
EDIT: It mounts a dmg called "InstallESD" with a bunch of packages in it.
