[RCU] MDB2 Error: constraint violation

Hi,

I'm trying to install roundcube from svn; this is what I get when I try to login (the login fails):

DB Error in /var/www/[redacted]/program/include/rcube_mdb2.php (718): MDB2 Error: constraint violation Query: _doQuery: [Error message: Could not execute statement] [Last executed query: EXECUTE mdb2_statement_mysql_3415882471425ebb3058fd14a2a2528d2e740add6 USING @0, @1, @2] [Native code: 1062] [Native message: Duplicate entry 'SJlwEcZI3UdK6f7r3,HcfYWdgsEJn2ghSoE6xWj7' for key 'PRIMARY']

I've set $rcmail_config['debug_level'] to 15 for this.

A backtrace indicates this originates in the shutdown routine (debug_backtrace() slightly trimmed for brevity):

[0] => Array
        [file] => /var/www/webmail2.ii.nl/program/include/rcube_mdb2.php
        [line] => 301
        [function] => _add_result

[1] => Array
        [file] => /var/www/webmail2.ii.nl/program/include/rcube_mdb2.php
        [line] => 219
        [function] => _query

[2] => Array
        [file] => /var/www/webmail2.ii.nl/program/includ/rcube_session.php
        [line] => 181
        [function] => query

[3] => Array
        [function] => db_write
        [class] => rcube_session

[4] => Array
        [file] => /var/www/webmail2.ii.nl/program/include/rcmail.php
        [line] => 1164
        [function] => session_write_close

[5] => Array
        [function] => shutdown

This is the only error I see (besides a lot of notices and strict errors when I set error_reporting to E_ALL); does anyone have a pointer where to start looking?

thanks, Mrten.