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If a trace is broken it must be inside the board. I could remove the two troublemaker CD3215s and check for continuity of the connections between XA and XB. Any pointers as to which lines are the important ones? I2C_UPC_XA_DBG_CTL? I2C_UPC_X?
I checked the lines between XA and XB (I2C_UPC_X) and they are all good. I did find two missing connections between XA and the redriver U3000 (I2C_UPC_X_DBG_CTL), but those should not matter for the inter-CD3215 communication, right?
I ended up checking all lines in the USB-C area that run between front and bottom logic board. I found 8 broken or intermittent lines, so this board surely has plenty of internal damage. I'm saying good-bye to it.