Cannot send session cache limiter - headers already sent - CodeIgniter (Session.php)

I know this is discussed already many times, but I think this is different case.

I am making a simple ajax call to a php method which is as below:

public function updateAbout()
{
    log_message('debug', "updateAbout is called", false);   

     log_message('debug',$this->input->post('fname'), false);

   log_message('debug', "updateAbout success", false);   

}

In this method I am simply trying to get data which is passed by ajax call.

But I don't get data posted by ajax call instead get 2 errors:

Severity: Warning --> session_start(): Cannot send session cache limiter - headers already sent ...\..\\libraries\Session\Session.php 143
Severity: Warning --> Cannot modify header information - headers already sent ..\..\libraries\Session\Session.php 171

Note:

I am not using session_start() anywhere in my project.

This error is pointing to session_start() method which is in Session.php file of CodeIgniter libraries. It is not pointing to anywhere in my code so I am not sure where do I look for issues.

I have already checked answers all over internet and none of them seem to solve this issue.

Why am I getting this error and how can I prevent this?

Edit:

As per suggestions in other answers, I have checked and removed spaces in <?php ?> in all my codes.

Here is my JavaScript code where ajx call is made.

      $("#btnupdateAbout").click(function(){

      $fname=$("#updatefname").val();
      $lname=$("#updatelname").val();
      $country=$("#updatecountry").val();
      $locality=$("#updatelocality").val();

      if($('#optradioMale').is(':checked')) { $gender="Male"; }
      else{$gender="Female";}

       $.ajax({

           url:"http://localhost/Voyager/ProfileControls/updateAbout",
           data:{'fname':$fname,'lname':$lname,'country':$country,'locality':$locality,'gender':$gender},
           method:"POST",
           contentType:false,
           cache:false,
           processData:false,

           success:function(){

           }

       });
   });