Comment 21 for bug 1068605

Solved this on my testing Ubuntu 20.04 LTS VM by installing greeter from MATE:

```
sudo apt-get purge arctica-greeter arctica-greeter-guest-session
sudo apt-get purge kylin-greeter ukui-greeter
sudo apt-get autoremove --purge

sudo apt-get install --reinstall ubuntu-mate-lightdm-theme
```

(see https://askubuntu.com/a/1233695/66509 )