there is something wrong with your template
look at the link that is is called and check the source code -- the html head etc is completely broken
www.airsoftarmas.es/tienda-online/user/editaddresscartBT.html?tmpl=component&flexible=register
e.g.
<!DOCTYPE HTML>
<html lang="es-es" dir="ltr" >
<head>
<m-eta charset="utf-8">
<jdoc:include type="head">
</head>
<body class="contentpane">
<div id="system-message-container">
</div>
<div id="FlexibleBilltoID" style="width:100%;margin:0px;">
<div id="checkoutbar" class="active" >
Registrar y comprar
</div>
<div class="checkout-content-cart">