How to fix this?

Error
Error messageDatabaseSchemaObjectExistsException: Cannot add index system_list to table system: index already exists. in DatabaseSchema_mysql->addIndex() (line 437 of /home/ns/www/www/includes/database/mysql/schema.inc).
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Uncaught exception thrown in session handler.

PDOException: SQLSTATE[42S22]: Column not found: 1054 Unknown column 'ssid' in 'where clause': SELECT 1 AS expression FROM {sessions} sessions WHERE ( (sid = :db_condition_placeholder_0) AND (ssid = :db_condition_placeholder_1) ); Array ( [:db_condition_placeholder_0] => af8b21c22a565597f223db686b32e5a1 [:db_condition_placeholder_1] => ) in _drupal_session_write() (line 209 of /home/ns/www/www/includes/session.inc).