“After about 3 seconds, the bootloader should turn on HDMI and displaying the loading sequence. Certain types of displays with odd resolutions are not supported at this stage and it might not display anything during this step and the next steps. If the Green LED turns green and nothing shows on your screen, please post your screen brand, model, and EDID here.”
Have you a different monitor you could try ?
Just out of interest - After the Le Potato has booted up for a minute or so - Have you tried Ctrl Alt F1 ?
Are you sure that the Astroberry image will work on the Le Potato ?