Yeah, I managed to detect one chip on the board. FINALLY :-).

Problem seems that the chip wasn't soldered properly. For this version, I first "reballed" the chip itself with some solder paste and then soldered it on the PCB. And now, I have a communication. The chip is responding!

But somehow, the current is very low and the chip seems not really hashing.

But I'm really happy that I can now talk to the ASIC ;-). Thank you all for your help. I really appreciate it!