Thank you for the answer, but I do not have a SD card in the slot.
What I realized in addition is that if I keep the Home button pressed for a while during switching on the Kobo the LED starts to flashing blue.
If I switch the Kobo on without
pressing Home button or through Paper Pin hole the blue light stays permanently on.
If I press through Paper Pin hole and switch on, nothings happens.
To sum up, I have the Kobo in a least 3 different states:
1. permanently blue light
2. flashing blue light
3. flashing red light
Are state 2 and 3 maybe some kind of boot modes for the Kobo or the IMX508? I suppose the permanently light is when the Kobo is running but the kernel stopped somewhere during start up.
Any sugesstion would be appreciated (since you are a Kobo Developer