Wouhouhou !!!
The problem is fixed by me !
My hosting manager made an upgrade of lightspeed (an alternative of Apache for people as me who didn’t know what it is) and this made the issue happen. He just made a downgrade and this solve the problem ! 
I still can’t tell what exactely is the problem, but the thing with the communication between the web-server and PHP seems to me to be the way to look in.
Thanks @rpacewicz for the posts very usefull to be sure that the problem came from the server, e
specialy this one :
Hope that it can help you and other people in the future !
(And hope you can understand my bad english of swiss french person)