Thanks for the reply. I think 2.4V is just fine because I have 2 of them in series and each chip should get its 1.2V operating voltage. But as I said, I measured an unequal distribution of the voltage.
Regarding cooling and the heatsink, I think you are totally right. I powered up the first board because I thought the chips would just getting warm as long as I don't let them hash. But I think I was wrong with that.

I have now assembled a couple more of my boards but on none of them I can detect the ASICs. I always get the message 0 ASICs detected but expecting 4.

And because of not getting any communication with the ASICs, I decided to have a board assembled with just 2 ASICs in parallel and supply them with 1V and 1.2V. But I have still the same issue that I don't get anything back from the chip and the chain is not detected.

I'm sending some data to the ASICs with 115200 baud on the UART with 1.8V level. The BI (busy input) pin is low. The 25MHz oscillator is running.
So, I'm slowly running out of ideas what I can do to get it up and running.

Has anybody an idea what else I can test?

Thanks!