In my case the serial number as the password worked. If it does not work
in your case, I suggest to open a ticket at Fortinet and call the
technical support.
I had the same Issue, what made it for me is this; 1-Boot the AP with
the console cable connected with 115200 baud with Putty. 2- At some
point the ap will offer you to press CTRL+C and then will ask you to
enter a password. The password is the seria...