In past there used the hack from TinyMCE editor so you might want to disable / uninstall Tinymce plugin and use another alternative. I am not 100% sure if this is same for Joomla TinyMCE.
Check the contents of .htaccess file for url redirection to their site if there is any redirection from your url.
Check to make sure all your plugins and modules are updated.
You can also speak to your host which they might have more information.
See you around...