Lack of program/localization directory (or its subfolders) or wrong access rights to it.
Do you have the folder /program/localization/en_US with the files labels.inc and messages.inc in it? What is your language set to in the RoundCube settings?
ALTER TABLE `messages` DROP INDEX `idx` ,DROP INDEX `uid` ;MySQL:#1091 - Can't DROP 'idx'; check that column/key exists
Is there anything in your error log?
[11-Jan-2011 20:18:38] MDB2 Error: no such table (-18): _doQuery: [Error message: Could not execute statement][Last executed query: PREPARE MDB2_STATEMENT_mysql_546a562dc4ab8881023350b62a1b6df6 FROM 'SELECT vars, ip, UNIX_TIMESTAMP(changed) AS changed FROM session WHERE sess_id = ?'][Native code: 1146][Native message: Table 'tobiasz_mail.session' doesn't exist][11-Jan-2011 20:18:38 +0100]: DB Error: _doQuery: [Error message: Could not execute statement][Last executed query: PREPARE MDB2_STATEMENT_mysql_546a562dc4ab8881023350b62a1b6df6 FROM 'SELECT vars, ip, UNIX_TIMESTAMP(changed) AS changed FROM session WHERE sess_id = ?'][Native code: 1146][Native message: Table 'tobiasz_mail.session' doesn't exist] in /home/tobiasz/public_html/program/include/rcube_mdb2.php on line 264 (GET /)