Install path is /var/lib/roundcube for Debian. Don't try to guess it.

Index: b/program/include/iniset.php
===================================================================
--- a/program/include/iniset.php	2010-07-17 17:33:04.000000000 +0200
+++ b/program/include/iniset.php	2010-07-17 17:33:23.000000000 +0200
@@ -27,7 +27,7 @@
 define('JS_OBJECT_NAME', 'rcmail');
 
 if (!defined('INSTALL_PATH')) {
-  define('INSTALL_PATH', dirname($_SERVER['SCRIPT_FILENAME']).'/');
+  define('INSTALL_PATH', '/var/lib/roundcube/');
 }
 
 define('RCMAIL_CONFIG_DIR', INSTALL_PATH . 'config');