Okay, I've got it. It only appears when I attach / detach the micro usb cable for loading the battery. When I remove the cable, the phone won't wake up again or (if the display was on) after the next sleep. Same thing happens when I attach the power cable.
I logged in via ssh and got this from syslog: https://paste.ubuntu.com/25171358/
After the last "handle_power_button_press" nothing happens - even when I press the button again (or press other buttons). SSH-connection is alive and the phone command line is usable.