Slackware Live Edition iso2usb.sh creates USB that fails to boot.

Hello I tried to make a USB with Slackware live edition as detailed below, but it does not boot every way I have tried:
1. UEFI mode with or without Secure Boot: In UEFI mode my laptop first says "verifying media", then it gives up on USB and boots off the hard drive.
2. BIOS/CSM mode gives the following failure messages.

SYSLINUX 6.04 EDD copyright 1994-2015 H. Peter Anvin et al
Failed to load libcom32.c32
Failed to load COM32 file vesamenu.c32
boot:
Failed to load libcom32.c32
Failed to load COM32 file vesamenu.c32
boot:
....(repeats periodically)

What do I do?

I thought I had successfully followed
https://docs.slackware.com/slackware:liveslak
downloaded slackware64-live-plasma5-current.iso
verified it with the .md5 and .asc files.

To make a bootable USB, I deleted all existing partitions from the USB, because otherwise the iso2usb.sh script would complain.

I used the following command:
iso2usb.sh -i /run/media/a/path/linux-slackware/slackware64-live-plasma5-current.iso -P -p persistance-slack -o /dev/sdc -

the output I will try to post in a followup message.