Unable to post a message to a website due to: "Exception": "Invalid CSRF token

I am unable to post a message at the following URL:

Edit: This is the URL again, but now changed so it does not become a Link:
boards.ie/post/discussion/help-desk#

When I click on their “Post Discussion” button, I receive the following error message:
{ “Code”: 403, “Exception”: “Invalid CSRF token. Please try again.”, “Class”: “Gdn_UserException” }

I tried clearing Cookies both on the PC and in Chrome, rebooting the computer and using Avast Secure Browser to access the same webpage, but to no effect - I receive the same error message.
Online information about the error message is that this error message indicates an “attack”.

I am using a PC running Windows 8.1 with Avast Free Antivirus, and Comodo Firewall.