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Booting hangs when external monitors are connected

When booting Ubuntu 20.04, the splashscreen hangs when an external monitor is connected.

In order to boot properly, I have to disconnect the external screen and reboot the system. I did not have this problem with previous Ubuntu releases (18.04, 19.10).

This is my configuration:

  • Thinkpad T460
  • Intel i7-6600U
  • GPU: Intel Corporation Skylake GT2 [HD Graphics 520]
  • HDMI connector

This is a follow-up of the issue at launchpad where other people have the problem with other and similar configurations.

It also describes a workaround:

Also, when I edit /etc/default/grub so that it reads
GRUB_CMDLINE_LINUX_DEFAULT=""
(that is, splash disabled)
it works fine with two external monitors attached.

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