Thank you, Jorge, but unfortunately, it still does not show any icon. Here a link to my site (still in maintenance mode) with Firebug Screenshot:
https://screencloud.net/v/i9zS
The function itself works, the site is opening in a new tab. Only the icons are not showing at all.
This is the source code of this site and part with wp-links:
https://screencloud.net/v/ETWI
I use WP 4.1 with ENFOLD theme, multisite, blogs in subdirectories and this one blog is mapped to another domain name.
When I take a look in my backend of WordPress to your plugin settings, the path of the icons, that I correctly see there, is:
https://mysite.de/wp-content/plugins/wp-links/icons/external-link-red-dark02.png
But actually, in my WordPress network installation, the original path of this sub-blog is https://mynewdomain.de/subblog1/
for example.
So I suppose that your plugin-URL path in wp-links.php is probably not working that way with
a) WordPress multisite together with
b) domain mapping
?