Ajax error HTTP Result Code: 200 CustomMessage: The response failed verification so will not be processed CMS: Drupal 7.41
The error means that X-Drupal-Ajax-Token header is being stripped between the server and browser on clients side.
This is normally means an over aggressive firewall on the client PC that strips the X-Drupal-Ajax-Token header causing the error.
To test the issue, disable the firewall & antivirus and then retrying the process. If the error goes away, try milder firewall settings. Should the error reoccur, please make a ticket with the exact steps to recreate the issue.