“Today I finally made the 24V connection and updated the FW via USB to version 2.14.” — does this mean that both 24VAC and USB are plugged in at the same time when you did the firmware upgrade?
If the controller can power on over USB, that means the microconotrller is alive. I am still a bit confused by your description: has the controller ever worked with 24VAC, or has it never? If it has never, you should check the orange terminal block and make sure the wires have good contacts with the terminal block pins. If it worked with 24VAC before but not now, perhaps the fuse is blown or other components were damaged.