Yes, I always press "u" and check if it is in Update Mode. No question is dumb in these thread.

Ok, I will try the different cable. Debians I use are different, but the same situatuin everywhere (when need to Cold Flash, still "no suitable device").

In fact, I see the problem in something "third". The Vanilla and Fiasco can be done quite without problems. But reflashing these two doesn't help and the problem is like before - boot looping with no result, restart, boot, nothing, restart, boot, nothing....Moreover the Cold Flash telling "no suitable device" - on Windows, and also on Ubuntu. What's the difference, between Vanilla/Fiasco flash and Cold Flash using Fiasco bin file?

What next is important to boot OS, instead eMMC, kernel?

What could cause loop booting with no result to infinity?