Spell check not working.

[User, Kolab]

Hi folks,

I run Roundcube Webmail 1.0-7.9.el6.kolab_3.1 and within the file 
/etc/roundcubemail/config.inc.php I only found these lines:

     $config['spellcheck_dictionary'] = true;
     $config['spellcheck_ignore_caps'] = true;
     $config['spellcheck_ignore_nums'] = true;
     $config['spellcheck_ignore_syms'] = true;

No errors within /var/log/roundcubemail/*

No changes after adding

     $config['spellcheck_uri'] = 
'https://spell.roundcube.net/tbproxy/spell?lang=';

ping https://spell.roundcube.net/tbproxy/spell?lang=
says ping: unknown host https://spell.roundcube.net/tbproxy/spell?lang=

How do I make sure the server is able to resolve/connect to 
https://spell.roundcube.net/tbproxy/spell?lang= ??

Any further ideas?

Thanks
ted