Hi everyone, and thanks a lot for the help.
I found the solution: the problem was Windows 11 on my laptop.
After writing the Armbian image, if the SD card stays inserted, it becomes corrupted within a few seconds.
This happens only with Armbian and only with R3S images. I have no idea why.
So, to get a proper SD card, I need quick hands to remove it immediately after the writing finishes (no matter which software is used to write).
