AC88U bricked (stuck at Starting program at 0x00008000)

My Asus AC88U is bricked,
I was able to connect a serial cable to it and could log the boot sequence
it is stuck at "Starting program at 0x00008000"

It does allow upgrading firmware (original Asus, Merlin etc.)
but it does not seem to pass this step.

I can interupt the bootloader: CFE 7.14.131.1608 (r589038)
and erase NVRAM etc.
but still no effect.

I have also tried loading an old version of the (Merlin) firmware (in case of incompatibility with the bootloader).
RT-AC88U_380.57_0.trx (I...

AC88U bricked (stuck at Starting program at 0x00008000)