I have the "Invalid Token" error issue after upgrading to v5.2.14. Submitted a support request on 12/5/13. I have cleared the cache in the system, cleared the cache in my browser (IE 10 and Chrome), and preformed a re-upload of the v5.2.14 installation files and still have the error. I am still waiting for a reply since yesterday. I am very frustrated as this error appears when I am trying to create a manual invoice. They seem to not seem to not think it is that important.
I am not a happy camper.
FYI - This quick fix does not work either: /http://www.randallkent.com/whmcs/whmcs-invalid-token-error-resolved