After some digging in my files, I found in an old(June 2020) backup of my savegame, so I gave it a try on the PTS, It worked!!!
Being very devoted into finding what's causing the savegame infinite loading in the PTS. I transfered gradually, piece-by-piece, parts of my main savegame in my old one...
So I began by selling all the trucks and every upgrades(garages, warehouse and on-maps) on both savegames.
After something line 50 attempts and 6 hours of success&fails, I figured that almost everything is ok in the savegame :
money, experience, map-data(discovered watchtowers, garages, trucks&upgrades) and profile data.
The only data that I couldn't transfer, as far as I can tell, are the Contracts and tasks.
So, the most obvious source of the problem is the last one.
I'm just pissed that I must lose 90% of my progress to play on the PTS...