I ran into a similar problem using YUMI-22.214.171.124 via wine-staging in Ubuntu 18.04.1. Hard to believe but some versions of YUMI do work in that configuration when the target (FAT32) USB stick is mounted prior to running it.
YUMI, however, works differently depending on circumstance. In Win 10 it extracted my xubuntu-18.04.1-desktop-amd64.iso into folders. In Linux, it just copied the iso to USB with this directory structure.
ubuntu.lst has these contents:
title Boot xubuntu-18.04.1-desktop-amd64
find --set-root %ISO%
map %ISO% (0xff)
kernel /casper/vmlinuz file=/cdrom/preseed/xubuntu.seed noprompt boot=casper iso-scan/filename=%ISO% quiet --
Notice that I commented out
initrd /casper/ILUG and replaced it with
initrd /casper/initrd. That solved the error for me. Presumably,
http://grub4dos.chenall.net/e/15 would describe error 15 as a missing
initrd. As of this writing, the page is not found.
I have ubuntu and lubuntu 18.04.1 on the same USB but their folders contain
initrd.lz and their
ubuntu.lst files say
initrd /casper/initrd.lz. For whatever reason, YUMI gets the correct, yet oddly named
initrd, but doesn't pass the correct filename to the resultant menu.
Not a grub4dos problem at all.