Topic: Unable to confirm security token.

When my client trying to register they are getting these error. Please help me how can I fix this issue. Thanks.

Please confirm or cancel your last action
Unable to confirm security token. A likely cause for this is that some time passed between when you first entered the page and when you submitted a form or clicked a link. If that is the case and you would like to continue with your action, please click the Confirm button. Otherwise, you should click the Cancel button to return to where you were.