You still didn't say what web browser you're using, other than you
aren't using IE 9.
I usually get diamonds directly through the website link and not through the in-game Diamond Buying button on the Item Shop interface. I just now tested the latter and no, it doesn't work as it should. It links to the old, and no longer active, Recharge Diamonds URL. So, the link that I provided in my previous message is the one which needs to be used. It can also be found on the main page under the Account tab.
The page I see when hitting that "Next" button after choosing the server is a white page... at the top in red is the URL: http://purchase.gamepay.de/xml/frontend.es_ES.psessionid=4f6e566e36a00f4180d95751 be94226a2af0beae/frontend1
Under that also in red labeled "sessionid" : 4f6e566e36a00f4180d95751be94226a2af0beae
Under that are several boxes with that lovely red "X" suggesting a picture that cannot be viewed where I can choose a language...
Under those there is, "Runes of Magic Play Now!" in black then under that underlined links giving me the option to Play... Recharge Diamonds... Home... News Archives...
All of those links are to the extreme left and set up like an index or the outline of a book... kind of like what you get if you have ever had FaceBook glitch on you.
It gives me this each and every time I try to get to the order page.
What you described above is exactly what I see when using IE 9, so it may be more of an issue with IE in general instead of specifically with IE 9. GM Nytefall and I have already tested IE 9 and he's reported the issue with it.
If you're using an earlier version of IE (IE 7 or IE

, I'd recommend downloading and installing a different browser for now and using it until the issues with IE can be examined and corrected. As I said previously, I use the Opera browser and use it almost exclusively. I've also used Firefox, even though my current installation is having some problems. While I do not use Google Chrome, you might also consider trying that browser.
Please let me know if any of this is helpful to you.