Upgrading to 21.04 leads to "screen lock requires gnome display manager" and random freezes while inactive
Solution 1:
Icons duplicated due to second "Desktop Icons" extension. To fix that, disable one in "GNOME Extensions" app. The newest one is called "Desktop Icons NG (DING)".
The problem with "screen lock requires gnome display manager" I solved by reconfiguring gdm by running "sudo dpkg-reconfigure gdm3" and selecting "gdm3" instead of "lightdm". It changed looks of login screen after reboot but now it is possible to lock desktop.