Ok, I solved the problem.
Here is what happened: I considered that I was the only one with this problem and, moreover, I had it with both of my devices.
The conclusion follows: the problem must be somewhere in my environment.
To make the long story short, I realized that the problem is in the way I transfer files from my pc to the boox. More precisely, since I use linux on my pc, I realized that the problem was in the way I mounted the boox on my pc: it was enough to pass "utf8" as a mount option to solve te problem!
The irony, in all of this, is that also the boox is run by linux but it communicates with the pc via a vfat filesystem!
Anyway, thank yours feedbacks have been precious, since they allowed me to focus the point: thank you.