It seems that the payment-site (at least the version for the Netherlands) doesn't support with IE9 (in other browsers, no problems occur).
You can generate the bug as follows:
In IE9, go to https://payments.wikimedia.org/index.php/Special:GlobalCollectGateway?uselang=en&form_name=RapidHtml&appeal=JimmyQuote&ffname=rtbt-ideal&payment_method=rtbt - fill in an amount and select a random Bank. The result is "Requested an unavailable or non-existent form." with some backtrack details. In other browsers, everything goes fine.
Version: master
Severity: major