3D FlipBook - 3D Flipbook displaying squares instead of correct glyphs

I bought a one site licence and made it run correctly on https://dev7.casi-regionlimoges.com/comite/pages_dyna.php?idpage=1029 Now on the prod website https://casi-regionlimoges.com/comite/pages_dyna.php?idpage=1029, some users claim they get Squares replacing every character. With some others, no problem !?! We are using the same Licence Key for both websites. What may be the issue? Yours faifhfully. P. BERTHET