I did following procedure:
1. Kobo is off, no SD card inserted.
2. switch on the Kobo -> permanent blue light
3. press HOME button
4. do PIN reset -> red flashing light
After more than 30 minutes the light is still flashing red. I don't think that a factory reset takes that long.
Pressing HOME button and doing PIN reset switches between red flashing light and blue flashing light and back again.
Is there a procedure to put the Kobo in USB boot mode? It seems that for the Kindle touch (also based on IMX508) such a procedure exists.