I bought the card a few years ago from a eBay seller. It worked alternately. Reading this topic I realized that the problem could be the SD Card in use, so I tried various brands. The latest from Samsung seemed to be fine except with the usual error "insert a FAT formatted SD Card ..."
Last week I bought a new KFF from TheFutureWas8Bit and this works smoothly.
But I wanted to recover the old one, but apparently it is not an easily solved problem.
Thanks to all for your help.