Open a second browser tab then log into the main website on that tab, After you log in on the main website, come back to this thread and click this link to make sure you are going to the correct Diamond Purchasing page...
http://us.runesofmagic.gameforge.com/user/payment , verify the link on the server selection page is the exact same as the link you just clicked and that you are not on the wrong countries page. When choosing your payment option make sure you are not clicking "Pay by Cell".
If the error is still occurring then clear your browser's cache and cookies, then try again, if it fails again then try another browser.
I checked 2 different pc's with different internet connection types, Cable/DSL and each one worked correctly while using Firefox web browser.