IF you use a browser with Greasemonkey/Tampermonkey support, you could try the following userscript, which works for me:
// ==UserScript==
// @name Opalstack - select DE server
// @namespace http://tampermonkey.net/
// @version 0.1
// @description try to take over the world!
// @author You
// @match https://webmail.opalstacked.com/*
// @grant none
// ==/UserScript==
(function() {
'use strict';
$('#rcmloginhost').val('ssl://mail.de.opalstack.com');
})();
Strictly a stopgap solution ;-)