RPi4: Viewsonic IFP-7550-2 is blanking when USB for touchscreen is connected

I have a RPi4B (4GB) connected to a Viewsonic IFP7550-2 touchscreen which is working fine at 4k/30Hz. When I connect the USB cable for the touch function, the screen goes blank. When unplugging again, the desktop comes back but the user is logged out and needs to login again.

To reproduce

1. Boot Raspbian to desktop with only HDMI connected.
2. Connect touch USB (-> screen blanks immediately)
3. Unplug touch USB (-> screen comes back with login popup)

Expected behaviour
Touchscreen schould be usable and not blank the screen.
(When using a Win10 PC instead of the Pi, everything works as expected!)

System

pi@raspi-ifp7550:~ $ cat /etc/os-release
PRETTY_NAME="Raspbian GNU/Linux 10 (buster)"
NAME="Raspbian GNU/Linux"
VERSION_ID="10"
VERSION="10 (buster)"
VERSION_CODENAME=buster
ID=raspbian
ID_LIKE=debian
HOME_URL="http://www.raspbian.org/"
SUPPORT_URL="http://www.raspbian.org/RaspbianForums"
BUG_REPORT_URL="http://www.raspbian.org/RaspbianBugs"
pi@raspi-ifp7550:~ $ cat /etc/rpi-issue
Raspberry Pi reference 2019-09-26
Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, 80d486687ea77d31fc3fc13cf3a2f8b464e129be, stage5
pi@raspi-ifp7550:~ $ vcgencmd version
Sep  9 2019 18:49:23
Copyright (c) 2012 Broadcom
version a62a87c77be35ef577563092923cf86e3d8b9408 (clean) (release) (start_db)
pi@raspi-ifp7550:~ $ uname -a
Linux raspi-ifp7550 4.19.71-v7l+ #1262 SMP Mon Sep 9 18:36:37 BST 2019 armv7l GNU/Linux

Reproduce the HDMI-only and touchscreen-USB connection sequence on an RPi4 using the listed Raspbian Buster and kernel versions, and review the system details from /etc/os-release, /etc/rpi-issue, vcgencmd version, and uname -a. Done means the touchscreen remains usable when connected and unplugging it does not blank the display or force a login.