All right !
Problem of images not displayed is solved, thank to you. I re-upload Hika 2.3.5 and it works fine.
But still have problem with this encode error.
Is there somewhere in Hika a parameter to fix encoding ? When I enable the native cache of J! in configuration+plugin, each page of the website is displayed properly, except product page. And no output message in Firebug.
Curiously, after the browser error message, it works when I reload the page.
Anyway, thanks a lot for solving the image problem ! And long live to Hika.