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Ok, I've found the solution.

This problem happens with ie8 users ... Our theme is not up to date...

in header.tpl change

{if isset($css_files)}
        {foreach from=$css_files key=css_uri item=media}
                <link rel="stylesheet" href="{$css_uri|escape:'html':'UTF-8'}" type="text/css" media="{$media|escape:'html':'UTF-8'}" />
        {/foreach}
{/if}

with (I've found it here ; https://github.com/PrestaShop/PrestaShop/blob/1.6.1.x/themes/default-bootstrap/header.tpl)

		{if isset($css_files)}
			{foreach from=$css_files key=css_uri item=media}
				{if $css_uri == 'lteIE9'}
					<!--[if lte IE 9]>
					{foreach from=$css_files[$css_uri] key=css_uriie9 item=mediaie9}
					<link rel="stylesheet" href="{$css_uriie9|escape:'html':'UTF-8'}" type="text/css" media="{$mediaie9|escape:'html':'UTF-8'}" />
					{/foreach}
					<![endif]-->
				{else}
					<link rel="stylesheet" href="{$css_uri|escape:'html':'UTF-8'}" type="text/css" media="{$media|escape:'html':'UTF-8'}" />
				{/if}
			{/foreach}
		{/if}