After enabling ssh, I could figure out that the xserver did not start.
startx said "no screens found".
So I tried out all the ways to config the pi in headless mode
Editing the config.txt for fixed hdmi modes (see here), fiddeling with raspi-config, updating the firmware - nothing worked.
Eventuall a look into the Xorg.0.log (in /var/log/) showed ...
It seems you have an entry in your /etc/apt/sources.list pointing to a repository at http://deb.playonlinux.com that doesn't support architecture 'armhf'. The Raspberry Pi has this architecture so you cannot use this repository. If so, then comment this line and redo:
rpi ~$ sudo apt update