I had the same problems as Lioelx. In my case it was that I used a different name for the database_optimizer table than the default. This is really a (minor) bug in the code.
In admin/include/modules/database_optimizer.php, change line 42 from
$query = mysql_query("select last_update from database_optimizer");
to
$query = mysql_query("select last_update from " . TABLE_DATABASE_OPTIMIZER);
In the same way, change all references (5) to database_optimizer in admin/include/database_optimzer_common.php
Jack_mcs, maybe you can fix this in the next release?
Regards,
/Martin