Issue: the Raspberry Pi (RPi) has crashed and the boot process ends up into a kernel panic. The reason is unclear. I am connecting the serial output from the RPi to a FTDI to see the issue on the screen. Which kind of FTDI or USB-to-serial board you are using is not important. A cheap model is fine. You must connect the FTDI pins to the RPi pins this way
Please refer to this resources for more information how to connect the serial output from the RPi:
Configure the RPi boot using the cmdline.txt file. My example: