[linkinsoldier]

When I use the "force HDMI Hotplug" option in config.txt, I get HDMI every time, however it only shows a "small picture in the middle of the screen". Its about 640p (instead of the choosen 1080p) - I guess there is a bug?

Did you follow my link? You need to do more than just hdmi_force_hotplug to get the right resolution.

Note: The issue is that kodi needs to read the EDID from the TV which contains the list of supported resolutions - and this isn't available when kodi boots.
You either need to store this information to a file (as described in my link) or slow down booting enough that the TV is ready. 

 I tried it and it works as well. So this is a known issue and there will be no improvement for "plug and play" user, right? Just asking because a friend of mine has the same issue and he has no idea of ssh. I will fix it for him, but still... i think there might be a lot of others out there.

Solution I and II which worked for me:

I) Solution: Boot up Delay:
1) ssh into rpi
ssh root@libreelec
2) By default /flash is mounted read-only so you need to:
mount -o remount,rw /flash
3) Edit the config.txt
nano /flash/config.txt
4) Introduce a new line for a boot up delay at the end of the config.txt:
boot_delay=20

Remark:here you have to do a "try and error". My android TV takes about something between 15-19s to boot up, therefore 20s works for me.

5) Save the change (CTLR+X -> Y -> Return -> Return)
Now it should work on the next boot!

II) Solution: Store the edid.dat and use this if no hotplug is present.
https://kodi.wiki/view/Raspberry_Pi_FAQ#...d_on_first

1) ssh into rpi
ssh root@libreelec
2) By default /flash is mounted read-only so you need to:
mount -o remount,rw /flash
3) If the display is now correct (precondition!) create edid Data for this HDMI connection
tvservice -d edid.dat
4) Copy the created edid.dat to /flash
cp edid.dat /flash/
5) Edit the config.txt
nano /flash/config.txt
6) Uncomment the line, so the HDMI connection is forced even if unplugged:
hdmi_force_hotplug=1
7) Introduce a new line to force using the edid data from the file:
hdmi_edid_file=1
8) Save the change (CTLR+X -> Y -> Return -> Return)
Now it should work on the next boot!

Remark: This only works for the setup you created the edid.dat with.
If there is any chance in your hardware setup, you have to redo steps 1-4!