It's the link in the invoice.
The fix consists in changing the code:
$js = 'parent.window.location.href=\''.$url.'redirect_popup=1\';';
echo '<html><head><script type="text/javascript">'.$js.'</script></head><body></body></html>';
by:
echo '<html><head>
<script type="text/javascript">
window.onload = function(){
window.location.href=\''.$url.'redirect_popup=1\';
setTimeout(function(){
window.parent.hikashop.closeBox();
},2000);
}
</script></head><body></body></html>';
in the file plugins/hikashop/attachinvoice/attachinvoice.php
So you can directly check the code in that file and change the code if necessary.