Fortinet have now released EMS 7.0.1
In the release notes it does mention a fix for the installer package not working, so I assume this fixes this.
I got around this by building new servers directly on version 7.0.0 as it was the upgrade that was causing the issue on some of my servers.
If I installed directly I didn't get the Invalid Installer error.
So you can either re-install you version or maybe go to 7.0.1