Hi
I've been having this error ever since i tried upgrading my roundcube and now no one can access emails Configuration Error defaults.inc.php not found
That file is a required file since 1.0.0. It should be in the config folder inside the Roundcube directory.
That may not be all you need - the old versions used db-inc.php for the database connection info and main.inc.php for all other config options. Neither file is used in version 1.0.x. The defaults.inc.php contains all available config options at their default value. All non-default values (including the database connection info) must be copied to config.inc.php.
In theory, you could simply edit the defaults file, but that's a bad practice. Having a file that contains only the custom and non-default options in it will make future upgrades simpler.