Error "POST" 500 with ajax request.

What does the <form> look like?

Yes sorry my mistake..
Here...

Code:

<?php echo form_open('users/create_user', 'class="customForm" id="userRegister"'); ?>
 <div class="row">
   <div class="col-lg">
     <div class="form-group">
       <select name="userGroupInput" id="userGroupInput" class="form-control" required>
         <option value="">-- Account type --</option>
         <option value="1">Group</option>
         <option value="2">Individual</option>
       </select>
     </div>
   </div>
   <div class="col-lg mx-0">
     <div class="form-group">
       <input type="text" class="form-control" name="cityInput" id="cityInput" placeholder="City" required>
     </div>
   </div>
 </div>
 <div class="row moreInfo hidden">
   <div class="col-lg">
     <div class="form-group">
       <input type="text" class="form-control" name="websiteInput" id="websiteInput" placeholder="Website">
     </div>
   </div>
 </div>

<div class="row">
    <div class="col-lg">
      <div class="form-group">
        <input type="password" class="form-control" name="passInput" minlength="4" id="passInput" placeholder="Password" required>
      </div>
    </div>
    <div class="col-lg">
      <div class="form-group">
        <input type="password" class="form-control" name="passInput2" minlength="4" id="passInput2" equalTo="#passInput" placeholder="Re-enter password" required>
      </div>
    </div>
  </div>

 <div class="row">
   <div class="col-md">
     <div class="form-group">
       <input type="email" class="form-control" name="email" id="email" placeholder="Email" required>
       <small class="text-danger text-center"><span id="email_result"></span></small>
     </div>
   </div>
   <div class="col-md">
<div class="form-group">
<input type="text" class="form-control" name="usernameInput" id="usernameInput" placeholder="Username" required>
</div[/font][/size]
   </div>
 </div>
 <div class="row">
   <div class="col-lg-2 mt-3">
     <button type="reset" form="userRegister" class="btn btn-danger btn-block"><i class="fas fa-broom"></i></button>
   </div>
   <div class="col-lg mt-3">
     <button type="submit" id="register_btn" form="userRegister" class="btn btn-custom btn-block "><i class="fas fa-pencil-alt"></i> Register</button>
   </div>
 </div>
</form>

I've included some of the inputs.. 


I've removed

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