From my side, this day starts very well!!!
It works !
The answer was here:
Did you already try the option to use the system’s dialogues for Open/Save?
See > Tools > Options > LibreOffic > General
I deselect the box. And it Works 
Very strange... It was the case with my previous Ubuntu
Nice day for everyone :).
Note 1: I select it again. Same issue. Files hidden.
Note 2: To do individualy for each LibreOffice application: Calc, writer...