Missing icons at the backend

If you would see a screen like this:

Or a javascript error like this:
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://xy.com/libraries/nextend2/nextend/media/fonts/Nextend.ttf?e06y41. (Reason: CORS header 'Access-Control-Allow-Origin' missing).

It means, that the fonts aren't correctly set up in your server. Open up your .htaccess file, and put these codes into it:

AddType font/ttf .ttf
AddType font/eot .eot
AddType font/otf .otf
AddType font/woff .woff
AddType font/woff2 .woff2
<FilesMatch "\.(ttf|otf|eot|woff|woff2)$">
    <IfModule mod_headers.c>
        Header set Access-Control-Allow-Origin "*"
    </IfModule>
</FilesMatch>

Which usually solves it, but if you are not sure about yourself, or this doesn't have an effect, you should contact your server host, and ask them for help, tell them, that your fonts aren't loading correctly.

HTML app

If you are using the HTML app, be sure to access the site using the link you entered to the config.php file. If it was a www URL, use it that way, if it was a non-www URL, use that, because some servers don't allow fonts to be used between www and non-www URLs.