I am using batch process to pass users to an external site using their API.

for each user I set $batch['operations'][] = array('process_funcion', array(LIST OF VARIABLES));

The batch processing works for upto 5000 users... anything over that I get

Access denied
You are not authorized to access this page. 

I thought this had to do with max_packet size... but I am not to sure of that anymore.

the batch is being initiated after a form is being processed.

any ideas on how I can process 5000 users at a time or why I am getting this access denied?

Would really appreciate the help :)

EDIT: I think that the insert to the batch table is to big, so I am trying to split this into separate batches like this:

$contactCount = 1;
      foreach ($contacts as $contact) {
        $batch['operations'][] = array('_contact_add_batch',array($module,$acc_id,$Group_ID,$verb,$contact));
        //Due to max_packet_size restrictions, need to set multiple batches
        if(($contactCount % 2000) == 0){
          batch_set($batch);
        }
        $contactCount++;
      }
        //For the last set of batches, in the event that there is not 2000, lets add them also
        if(!empty($batch['operations'])) {
        batch_set($batch);
        }
      }

but I am still getting the Access Denied error