I just found out how to disable gzip compression which is by default enabled on ovh web hosting.
We have simply to add the 3 following lines to the .htaccess file which is at the root of the nextcloud directory:
SetEnv no-gzip 1
And, as davDroid indicated, it works perfectly once this gzip compression is disabled.
Again thank you so much for all your replies!
PS : I don’t know how to mark this topic to “solved” but it is solved.