I have a similar issue (or need for education).
I receive the same "CPU Error Detected" dialog although I am able to ping the Panel IP. This is a USP-156-B10. I've successfully upgraded the firmware to 1.26.91 using the Copy to DOK method with a 32gb usb drive.
I have no problem downloading my program to the PLC and the expected IP address settings for both the Panel IP and the CPU IP are present. But my CPU IP does not have a MAC address.
My goal is to establish TCP/IP communication with the CPU.
I have been educated. I thought this was an all in one but we didn't buy the CPU unit that goes on the back. Pretty simple once you've got that.