Someone who reported having the same problems (on the odroid forums) says it works with coreelec 9.0.1. (LibreELEC 9, odroid-c2: device does not get powered on by TV with CEC - ODROID)
Switched to coreelec 9.0.1 simply by putting the coreelec update into the Update folder an then did a reboot.
All Problems gone,
CEC Poweron works
no flashing white bars on the right side of the screen anymore
no sudden stops on live tv or recordings
no skip problems anymore