class.config.php reads in the configuration from the ost_config table.
If it is saying undefined then I would guess its not getting the information from that table like it should be.
Whats the id of the record in your ost_config table??
Does settings.php -or- ost-config.php have the proper
define('TABLE_PREFIX','ost_');
ie does that match how your tables are setup in the db?