(Thinkpad L480) Booting hangs at the spinner screen when an HDMI cable is connected to an external monitor

Laptop: Lenovo ThinkPad L480
External monitor: Philips 276E8VJSB (4K UHD)

Steps to reproduce the problem:

1. While turned off, connect an HDMI cable between the laptop and an external monitor
2. Turn on the laptop
3. Booting hangs at the spinner screen, where it's written "Press Ctrl+C to skip"

The laptop boots up normally when the HDMI cable is not connected.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: linux-image-5.4.0-42-generic 5.4.0-42.46
ProcVersionSignature: Ubuntu 5.4.0-42.46-generic 5.4.44
Uname: Linux 5.4.0-42-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.4
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: amr 1875 F.... pulseaudio
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Thu Jul 23 16:08:14 2020
InstallationDate: Installed on 2020-04-26 (88 days ago)
InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
Lsusb:
 Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
 Bus 001 Device 004: ID 5986:2113 Acer, Inc Integrated Camera
 Bus 001 Device 002: ID 0461:4e57 Primax Electronics, Ltd HID compliant device
 Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
MachineType: LENOVO 20LTS01800
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
ProcFB: 0 i915drmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.4.0-42-generic root=UUID=a25a653c-6a2a-4954-bd98-bfe2582a2b74 ro quiet splash vt.handoff=7
RelatedPackageVersions:
 linux-restricted-modules-5.4.0-42-generic N/A
 linux-backports-modules-5.4.0-42-generic N/A
 linux-firmware 1.187.1
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 12/18/2019
dmi.bios.vendor: LENOVO
dmi.bios.version: R0QET60W (1.37 )
dmi.board.asset.tag: Not Available
dmi.board.name: 20LTS01800
dmi.board.vendor: LENOVO
dmi.board.version: Not Defined
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: None
dmi.modalias: dmi:bvnLENOVO:bvrR0QET60W(1.37):bd12/18/2019:svnLENOVO:pn20LTS01800:pvrThinkPadL480:rvnLENOVO:rn20LTS01800:rvrNotDefined:cvnLENOVO:ct10:cvrNone:
dmi.product.family: ThinkPad L480
dmi.product.name: 20LTS01800
dmi.product.sku: LENOVO_MT_20LT_BU_Think_FM_ThinkPad L480
dmi.product.version: ThinkPad L480
dmi.sys.vendor: LENOVO