@Gianluca Hi, I have watched your videos, and also tested updating here on a v1.2a 1LC.
One thing I noticed is that if I disconnect the CAN cable, then I hold down the buttons and power up, I get an initial flash of the STATUS LED (like yours does), then let go of the buttons after a few seconds, I get a constant flashing on the red STATUS LED. I think the bootloader falls back to the firmware if it doesn't receive a response from the mainboard, but yours is then crashing. This makes me think that the firmware on your board is corrupted, and possibly stuck in a boot loop. However, pressing the two buttons on power up should allow it to request a new firmware, even if the current firmware is corrupt. Which makes me think there is a problem with the CAN cable, but it's possible there is some other fault with the board.
With the 1LC not finding a CAN connection, I think the main issue is with the CAN cable. Even with no firmware, or when pressing the buttons, you should still get a CAN connection. You said you replaced the cable, but to me it appears to still be a problem. Please try making a new cable. Do you have a working toolboard at the moment that you can test with?
Also, check your CAN termination. I see you have a resistor connected to the CAN connector on the toolboard, does that mean you have NOT soldered the CAN termination jumper on the back of the board? You shouldn't do both. We recommend bridging the CAN termination jumper on the back of the board, see https://docs.duet3d.com/Duet3D_hardware/Duet_3_family/Duet_3_Toolboard_1LC#terminating-resistor
If you stick with the resistor, make sure it is properly connected and providing resistance. Measure the resistance on the pins of the CAN cable connector.
I'm not sure what was causing the initial problem of the memory error, but hopefully we can get back to that.
Ian