If you are Joomla user or designer/developer, I am sure that you will at least one time come a cross 'Invalid Token' message. I also used to get this error message and when I Google, I can to know that this is Joomla's security tokens to forms. This token prevent our Joomla site from most Cross Site Request Forgery (CSRF) attacks. You can read full article here.
If you read the above article there's a different way you can solve the problem. The easiest way to avoid this error message is to have your own custom message using 'Invalid Token Interceptor' plugin.
You can download the Plugin here.