thank you so much for reply! I bricked my Raduino while installing an AGC board to my uBitx, there was a short on my AGC PCB where I hasn't scratched properly. Unfortunately there got battery power to the 5V pin of the Arduino over this short.
Before the incident, I used also the firmware from KD8CEC on my uBitx and it worked perfect. The part number of my replacement chip is SI5351A-B04486-GT. I bought it from some UK based eBay store. The replacement of the chip was quite easy for me since I'm used to solder really small SMD packages. So all pins are soldered properly. There were signals on SDA and SCL.
While looking for some solution for my problem I found this thread here: https://www.silabs.com/community/timing/forum.topic.html/si5351_-_strangebeh-op0N
This guy seemed to have the same problem as me. And he got a solution for it. The I2C address of this particular chip is 0x62 instead of 0x60. Compiling the KD8CEC firmware with this small modification, my Raduino started to work as I expected.
But now I have some other problems with my uBitx. It seems to receive very bad. Only a few strong signals will be hearable. I also wondered about the signal level, my SI5351 is generating. While CLK2 has a level of 0.4Vpp on 160m band, on 10m the output is only about 0.1Vpp and I don't know if this should be like this. Also on the lower bands I receive stronger background noise, but no noise on everything above 20MHz.
I calibrated the main calibration with my frequency counter (soviet Ч3-54, very accurate and it has nixie tubes :-) ). I adjusted CLK2 at my raduino to be 55.000MHz when 10.000MHz is tuned.
Also my BFO seems to need a calibration. The signal quality of the few signals I can receive, is very bad.