This must be an code that was mean for testing only. It must be commented out because it prevent A2Billing to use any server group different than the 1st one. And if the administrator deletes the 1st server group by mistake or on purpose, than he/she will never be able to use the callback daemon unless he/she manually edit the database to set the id field of one of the groups to 1 and update cc_server_manager accordingly.
In the callback daemon, I have trace the error to the code on line 240
$id_server_group = 1;
This should be commented out or totally removed cause the $id_server_group is to be populated from the SQL Query ran earlier.
#$id_server_group = 1;
