Hi,
at first check http error log. Then check htaccess files (there is one in modules/CRM/Roundcube/RC also) and try to remove them. Also some server configurations don't allow to set 777 on files, but then you should get 500 internal server error.
Try to create a simple script in /modules/CRM/Roundcube/RC/ like test.php
<?php
echo "test";
and try to open that in the browser.
Regards,
Adam
