Why is $this->input->post('XYZ') returning empty in CodeIgniter?

I'm encountering an issue with CodeIgniter3.1 Version. The $this->input->post('XYZ') always returns empty when I submit a form or make an AJAX call. Strangely, it works fine on my local machine but not on the server. I have set csrf_protection to FALSE and there is a .htaccess file in place. The following is my form code, and it behaves the same whether I use "form_open" or input the code directly. Why is it only working on my local machine?

html
<form method="post" id="login_form" action="/index.php/login">
    <label for="email"><?php echo $this->lang->line('username'); ?> *</label>
    <input type="text" name="email" id="email" class="logfeld" autocomplete="username" />
    <label for="passwort"><?php echo $this->lang->line('password'); ?> *</label>
    <input type="password" name="passwort" id="passwort" class="logfeld" autocomplete="current-password" />
    <input class="sendbut" type="submit" name="login" value="<?php echo set_value('passwort'); ?>" />
</form>