The website works fine for me. When a site gives a validation error, it usually means it cannot authenticate the security ticket. If you notice the URL starts with “https”, this means the website is trying to use a secure connection and your browser will try to authenticate it.
Depending on the browser you’re using, you can add an exception, essentially telling the browser to ignore the lack of an authenticated security ticket and continue on anyway. This isn’t advisable for sites where personal information (i.e. credit card numbers) may be exchanged, however.
Some common fixes involve rebooting your computer or resetting your internet connection. Firewall and router settings can also cause such mischief, but solutions involving these differ greatly depending on software/hardware brands.