I've had a problem with the Ubuntu Server installer for a number of versions now. USB keyboards just don't work in it with USB Legacy enabled. I suspect it works with Legacy disabled, but I can't get that far since GRUB doesn't work with Legacy disabled so I can't get in to the installer.
Has anyone been able to work around this? It's not a problem with the keyboards or the computers. Multiple keyboards, multiple machines, multiple versions of Server installer all exhibit the same behavior. CentOS's installer works fine with Legacy enabled, so it's something specific to Ubuntu's Server installer.