Invalid CSRF token when submitting a form

PHP Code:

   <?php echo form_open(site_url('stock/search')); ?>

      <input type="text" class="searchbox" id="searchbox" name="searchTerm" placeholder="Search">


<?php echo form_close();?>

Gives the same error:

Invalid CSRF token.

I have noticed that if I get the above error and hit refresh I get:

An Error Was Encountered

The action you have requested is not allowed.

looking in the log file, i can't see anything to indicate whats going wrong:

INFO - 24-01-2018 15:06:43 --> Config Class Initialized
INFO - 24-01-2018 15:06:43 --> Hooks Class Initialized
DEBUG - 24-01-2018 15:06:43 --> UTF-8 Support Enabled
INFO - 24-01-2018 15:06:43 --> Utf8 Class Initialized
INFO - 24-01-2018 15:06:43 --> URI Class Initialized
INFO - 24-01-2018 15:06:43 --> Router Class Initialized
INFO - 24-01-2018 15:06:43 --> Output Class Initialized
INFO - 24-01-2018 15:06:43 --> Security Class Initialized
DEBUG - 24-01-2018 15:06:43 --> Global POST, GET and COOKIE data sanitized
INFO - 24-01-2018 15:06:43 --> CSRF cookie sent

Thank you for your help so far, it really has stumped me!