GDM3 does not start at system boot after upgrade from 18.04 to 20.04

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After the upgrade, I get the default logon screen (not gdm3). From a terminal, when I issue

sudo systemctl status gdm

it says "dead". Then, if I issue:

sudo systemctl restart gdm3

the GDM login screen comes up fine.
How can I figure out what is causing GDM to fail during system boot?

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