Long story short, during the download, 2 out of 2700 files, the wp-includes/http.php file and wp-admin/users.php to be exact, were downloaded as an empty file (0 bytes), while in the server their size is 16.787 bytes and 16.863 bytes respectively.
Instead, during the upload, 1 out of 2700 files, wasn't even uploaded, a file "flexslider.css" of 3.533 bytes of the Wordpress theme in use.
Now, I think this is the first time this is happened to me in so many (5~10) years of work. I can think of some subjects accountable of those errors:
1. bad shared hostings from which I downloaded/uploaded
2. my connection (DSL fiber 30MB/3MB)
3. my 2 months old new PC with Windows 10 Pro 64bit, 16GB ram
4. the new latest FileZilla client updated some days ago
5. random shit
Before opening a bug for this, I'll do some more test in the next days downloading lots of files from a reliable server to see if it happens again.
FileZilla Client
----------------
Version: 3.14.0
Build information:
Compiled for: x86_64-w64-mingw32
Compiled on: x86_64-unknown-linux-gnu
Build date: 2015-09-16
Compiled with: x86_64-w64-mingw32-gcc (GCC) 4.9.1
Compiler flags: -g -O2 -Wall -g -std=gnu++14
Linked against:
wxWidgets: 3.0.3
GnuTLS: 3.4.4
SQLite: 3.8.11.1
Operating system:
Name: Windows 10 (build 10240), 64-bit edition
Version: 10.0
Platform: 64-bit system
CPU features: sse sse2 sse3 ssse3 sse4.1 sse4.2 avx avx2 aes pclmulqdq rdrnd bmi2 bmi2
Settings dir: C:\Users\banca\AppData\Roaming\FileZilla\