My laptop (Dell XPS 13, 9350) has a single USB-C port, which I am used to connecting to a docking station which is in turn connected to peripherals (USB devices, audio, display, power supply). One day when booting up the computer, the display had stopped working: the laptop still 'sees' that an external monitor is connected, but the monitor reports that it receives no signal.
Since monitor and docking station were working fine, I feared the worst: a broken USB-C port. But when I booted my laptop from a live USB with Ubuntu, suddenly the external monitor was working. So the issue must be driver related. The problem started a few weeks ago and I honestly did not expect this issue to be driver related, so I am afraid I cannot reconstuct which changes were made to the software shortly before the issue started occurring.
In summary: Dell XPS 13 9350, Ubuntu 20.04.3, 64bit, kernel 5.11.0-27-generic, integrated Intel HD Graphics 520. USB-C port still works for power supply and to connect USB and audio devices, but no longer properly sends the Displayport signal.
Update: Based on the following log entry
i915 0000:00:02.0: [drm] *ERROR* Link Training Unsuccessful
I found the following related bug report on Launchpad. Supposedly, the problem should be solved in kernel version 5.11.
Curiously, I found that booting in the old kernel 5.4.0-81-generic instead resolved the issue. I would rather not downgrade so many kernel versions, and my technical knowledge is too limited to read from the discussion on the Launchpad bug report what to do to patch the problem manually.
