I think I found the faulty part. It's indeed the buffer chip. (Hex Inverter lvx04)
Voltage at the input pin (9) is 3.3V, and at the output pin (8) also 3.3V instead of 0V.

But now the big question. Where do I get such a replacement buffer? Is this a standart part?
The other possibility: This buffer chip has 6 inverters inside, I think only one of them is faulty. Question to the empeg team. Are all of them used, or is it possible to extra wire another pin combination?