Does anyone know where one can find an undamaged copy of the Mojave installer?

Of course it is not damaged. The App Store has detected a copy of Install macOS Mojave in your Applications folder.

Move it to the Trash, empty the Trash.

Connect your mac to your router via cable, restart your mac and try again.

If you have any existing copy of MacOS Installers in Applications folder please trash it.

Please use safari to open the link to download from the Apple support articles.

Hope it helps.