I found several topics which describe the same issue as I have but none of the solutions worked for me so far. So on boot, the raspberry asks for a username and password (what it shouldn't) and when I enter pi/raspberry the screen quickly gets black and then the login re-appears. This happened after I upgraded my Jessie to Stretch but if I'm correct, rebooting worked a few times.
I can still login through SSH so my credentials are correct.
As far as I can tell I only added one line in my .profile
which I reverted back already - without change.
The only hint I got so far are the following lines from my .xsession-errors
dbus-update-activation-environment: warning: error sending to systemd: org.freedesktop.DBus.Error.Spawn.ChildExited: Process org.freedesktop.systemd1 exited with status 1
/usr/bin/x-session-manager: 33: exec: /usr/bin/lxsession: not found
I'm not super experienced with Linux so I'm not sure if just re-installing LXDE would be the right thing here.