Good evening CiccioCB
As the ESP32 GPIO15 must be HIGH at boot, it seems to be very important to pull this pin up if GPIO15 is connected to any external medium or low resistance circuit that may draw it down at boot time. Otherwise the ESP32 can not start.
I had this problem e.g. when connecting a MAX7219-dot-matrix-display to an ESP32 devkit V4
There are some example circuits in the ANNEX32 help file using GPIO15 as the CS (ChipSelect) without an additional pullup resistor..
I suggest adding a pullup resistor of 10K in the documentation to insure the functionality.