Call to undefined function db_set_active()
The 2 things that come to mind: your installation is  not bootstrapping properly (improperly configured .htaccess or settings.php) , or you installation is corrupt.

I would recheck the apache logs, watchdog logs to see if any errors are mentionned, if all fails I would see if the application works with this module disabled, because this function is defined in core on line 2519 in includes/database/database.inc and should be available from all your custom module code.

Hope this helps, cheers!