Hi Rui,
the problem was finally solved by WPML support.
They made a small change in a file theme:
/wp-content/themes/dante/includes/plugins/aq_resizer-1x.php
In line 14 they changed from:
$url = $sitepress->convert_url($url,$sitepress->get_default_language());
to:
// $url = $sitepress->convert_url($url,$sitepress->get_default_language());
I think they are going to contact your staff to solve the problem for the future.