Missing parameter error on boot from Ubuntu startup disk

Problem

I made a startup USB flash drive for Ubuntu 15.04 using Startup Disk Creator as described here. I booted a computer using this flash drive and the boot process threw this error:

Missing parameter in configuration file. Keyword: path
gfxboot.c32: not a COM32R image
boot:

(This error happens even if the drive is created using UNetBootIn as described here. It happens with Ubuntu 14.10 too.)

Solution

Type help at the boot prompt. It will show a menu of options. Hit Enter here and Ubuntu will boot up as it should have.