The invalid security token error 403 is mostly known as the Forbidden error of an HTTP status code indicating that accessing that particular page or resource is absolutely forbidden for some reason.
The solutions which can move you forward are:
Checking for URL errors and carefully specifying the exact web page file name with its proper extension, and making sure it is not just a directory. In most cases the websites are configured to prohibit directory browsing, so you may get a 403 Forbidden message if it is trying to display a folder rather than a specific page and in that case what you are facing is normal and an expected behavior.
So it is necessary that while operating for the website in question, you can only prevent 403 errors, by enabling the directory browsing in the web server software you are using.
Clearing your browser's cache is definitely one of the required action, but as you said you have already done it, there's nothing more to talk about it, except that you keep repeating this.
Now if you are not able to Log in to the website because of a 403 Forbidden message it could mean it requires some additional access components to display the page.
Though generally, a website will show a 401 Unauthorized error when it requires special permission, but sometimes they might also give out a 403 Forbidden message instead.
Clearing cookies which you have already done is another option but make sure to enable them in your browser, or specially for this website to access the log in page. The 403 Forbidden error is a special indication that cookies might be one of the reasons behind this restricted access.
If nothing works, you need to Contact the website directly. It happens at times that the 403 Forbidden error is just getting displayed by mistake, though everybody who are trying to access it are facing it, the website people aren't yet aware of this problem.
Try to access this particular website from some other computer than yours and if that doesn't show up this error, then contact your ISP.
If you are getting this 403 message on all pages or for more sites it might be a possibility that your public IP address, or even your entire Internet Service Provider, has been blacklisted. This is also a situation which shows a 403 Forbidden error.
Visit the page later after verifying the correct HTTP and making sure that this 403 error is been displayed in other computers too, and you need to try until the problem gets resolved by the webmasters.