True. As I wrote, the router is softbricked. In this state, the above log can be read on the COM port.
I followed this description and I found that the upload is completed, the fip file is evaluated as uploaded (the lower log shows this), but after about 15-20 seconds a "resetting" line appears and the Hello BL31!!! loop appears, as in the upper log. I interpret this as meaning that the upload is successful, but it does not execute the contents of the fip file (network configuration for tftp) and the router restarts itself. But why?
What method can I use to find out?
Now I'm trying with a serial sniffer.