Checkout Page Problem - Pop up screen

Im having problems with my checkout page again.

Errors occour when clicking the buttons on the checkout page.

On the screen cast below, I spotted 3 errors

1. Big pop up box with html contained
2. Small pop up error box
3. Shipping method not selected, but it was selected.

Many of these errors happen if button is double clicked... which many users do... although you dont need to.

http://www.screenr.com/s118

Open cart Version 1.5.2.1 Fresh install with no addons. Out of the box!

Seems you may have missing files - re-upload via FTP

Thanks Simon. I have re-uploaded all files. But the error still appears. The error shows index.php missing.... but its NOT. It happens when the button is doulbe clicked!

Link to your site would be helpful.